SPAIN'S DAYS as a mecca for tourists wanting a slice of old Europe at old European prices are on the wane. Hotel accommodations there are expected to cost Americans as much as 50 percent more in 1979 than in 1978.

The latest of the reforms resulting from the demise of dictator Francisco Franco three years ago is the end of strict government controls on hotel prices. Hotels still must file the rates they intend to charge during a coming year with the Ministry of Information and Tourism by Nov. 1 of the preceding year, but government limits on the prices particular hotels may charge are gone.

One travel agent, when informed of the jump, responded: "There goes Spain. It's going to go the way of England, Germany and France" -- which may be a bit heavy on the gloom and doom, though many Americans believe those three countries are too costly to visit these days. The European Travel Commission, remaining optimistic, recently forecast a 6 percent rise in the number of Americans who will visit Europe this year over the 1978 total, which was 4 million.

It's too early to gauge the precise effect of the deregulation, but spokesmen for Spain's tourism ministry estimate a deluxe five-star hotel that cost $50 a night in 1978 will soon cost about $75. This is due in part to the devalued dollar, but results primarily from hoteliers being released from a price structure they had complained was unrealistic in today's inflationridden world.

Deluxe and four-star hotels were expected to order less of an increase than the three-star class hotels, which are most popular with the budget-minded travelers who also value comfort.

But count your blessings. Tourists traveling to Spain came close to losing the most charming chunk of one of the world's finest tourist bargains, the governmentrun system of hotels known as paradores (literally "stopping places" in Spanish).

At one time, some of the smallest of these hotels -- such as the six-room parador in the tower of a 14th-century castle at Villalba, the unbelievably classic 11-room one in the 12th-century castle of Alarcon, the 16-room parador in the surviving half of a 10th-century castle at Fuenterrabia, the beautiful 19-room, 15th-century palace at Zamora and others of similar inefficient petiteness -- were to be closed.

The impetus had come from Spaniards who complained that the government was, in effect, subsidizing the touists by operating hotels of such small size. These paradores generally had more employes than paying guests, or stayed open year-round when they often were filled only during the high season. Indeed, the paradores were such that only a government could own and operate them.

Since the beginning of the parador system in 1928, when the last king, Alphonso XIII, opened his royal hunting lodge at Gredos, west of Madrid, to the paying public, the Spanish government has sunk millions into the parador system. It has achieved consistently spectacular results. The paradores originally were located in remote sections of the country or in popular areas that lacked adequate accommodations.

Architects with exquisite taste and unlimited funds to draw upon were able to convert castles, or parts of crumbling castles or palaces, into hotels that have all the modern conveniences but still preserve the aura of their often stunning or historic surroundings. Modern additions usually blend in so well with the original structure that it is difficult to discern which parts are new. Genuine antique furniture is liberally sprinkled among period reproductions in the rooms and public salons of the paradores.

Thus, a room in the parador located in the castle at Alarcon, built on a high point in the loop of the River Jucar, will have for a window an ancient narrow slit about three inches wide in a wall four feet thick.

A reconstructed palace adjoining the strikingly huge, crenelated walls of Avila requires the guests to walk through a labyrinth of hallways to reach a room overlooking a well-manicured rose garden planted at the foot of the walls.

Paradores also were located in newly built structures, such as the one across the river from Toledo, on the site El Greco must have used to paint his views of the city. To afford each guest the same view El Greco had, each of the parador's 18 rooms has a balcony overlooking the well-preserved city.

The government built some large paradores that could be considered attractions in themselves, such as the 127-room one at Fuente De at the foot of the astoundingly beautiful Picos de Europa near the northern coast. Across the road from this parador is a funicular that carries passengers from the balmy valley to the top of the mountain, where they can literally throw snowballs at the mountain goats grazing there.

Paradores also were built high up in the mountains to accommodate hunters, such as the rustic-looking parador at Cazorla, atop the Sierra Nevada range east of Granada.

Where a castle was available but impractical to convert, paradores complete with swimming pools and other modern amenities were built nearby, with a perfect view of the picturesque castles. Such is the 23-room parador at Verin, just north of the Portuguese border.

Other paradores are noteworthy for their collection of antique furniture, particularly the one in a 12th-century hospital for pilgrims at Santo Domingo de la Calzada. It is filled with suits of armor, copper caldrons, and numerous centuries-old chests, tables and sideboards.

The most popular parador of all, however, is the one within the walls of the Alhambra at Granada, in what was a 15th-century convent. This, the second Spanish parador, will celebrate its 50th anniversary as such in 1979.

Although the 52 rooms in the Parador Nacional San Francisco in the Alhambra are filled practically all year and that parador turns a profit, Spanish critics of the system cited statistics that showed that more than half the paradores regularly lost money. The Ministry of information and Tourism gave in to the complaints and made plans this past year to close the costiliest paradores, ones with fewer rooms than employes. An albergue (Spanish motel), part of the parador system, was even closed at Pajares in northern Spain.

But after making plans for the closings, the government heard from townspeople where many of the small paradores had been purposely located. Many of the towns in remote sections of the country had come to depend upon the government hotels for employment and increasing tourism.

The tourist ministry then backed off, pondered the situation, and came up with a solution that should please all. Since the paradores were so popular it often was impossible for many travelers to get into them during the heavy tourist season, the government would add rooms to the smaller ones.

The first expansion, the seven-room Castillo de Santa Catalina at Jaen, was begun in 1977. The parador recently reopened with 15 rooms. The albergue at Pajares probably will be reopened after more rooms are added.

So, with the paradores to remain open, the largest remaining problem -- especially for American tourists -- is getting reservations at these fascinating accommodations.

As things now stand, the only practical way to get into a pardor is to leave your schedule flexible until you arrive in Spain. Call long distance from your hotel four or five days ahead to get a reservation (a 100-peseta tip, or about $1.50, to the desk clerk should easily get the job done for you).

Trying to make reservations from the United States usually is fruitless and costly. Most of the paradores simply will not respond to overseas requests for reservations, even if the person seeking the reservation spends the money on a costly cable and repays a reply. Each parador is managed separately, and although the service on the spot is invariably attentive, the government employes who manage these hotels aren't too disturbed by the prospect of rooms going empty.

De Carlo Tours in New York once specialized in parador reservations for Pan American Fly/Drive packages, and Iberia Airlines attempted it two years ago. But De Carlo booked its. packages through the Ministry of Tourism, not through individual paradores, where they also experienced difficulty in getting reservations (and still have problems when they occasionally try to make an individual booking). Neither Iberia nor De Carlo are currently offering parador packages because other packages constitute a more profitable use of their time. This is why you will rarely, if ever, find a parador stay included in a package tour. Thus, the paradores are for people who travel on their own, and the remoteness usually makes a car mandatory.

The Spanish Tourism Administration (ATE) in Madrid, which manages the parador system, claims it has noneed to make it easier for foreigners to get parador reservations. The idea behind the parador system, officials argue, was to provide a stopping place for travelers in areas where accommodations were meager or poor and in areas that needed an infusion of tourist money.

What that viewpoint ignores, however, is that because of the uniqueness of the paradores, they have become attractions in themselves. The parador at Fuente De, for example, is not a place you happen to pass by. The road to it winds through rural valleys, across mountains and dead ends just beyond the parador. There is no way the parador can be considered merely a casual stopping place.

The other government argument is that the paradors also are popular with Spaniards and should serve the Spaniards first. If foreigners could easily make reservations before arriving in Spain, they could book all the parador rooms and Spaniards could never get in for a weekend stay unless they planned months in advance. In fact, however, in some small popular paradores, Spaniards do telephone and book most of the rooms far in advance.

Despite the controversy and frustrations, the system is growing. At last count there were 70 paradores (42 four-stars, 27 three-stars and 1 two-star), eight albergues and three hosterias (restaurants with no sleeping accommodations). In addition, the Hotel Atlantico in Cadiz is a privately operated but government-owned hotel and seven "paradores colaboradores," are privately owned but run in conjunction with ATE.

New paradores are scheduled to open within the next year at Almagro, in central Spain; at Seo de Urgel, in the Pyrennes on the way to Andorra, and in the coastal town of Lequeitio and at Elorrio, both in the troubled Basque region.

It has been our experience, too, that all the paradores are underrated by the government's one-to five-star rating system. Each parador should be rated at least one star higher than it is, judging by equivalent accommodations in regular hotels. But with the rating system, the government in the past has been able to hold down the prices, offering good competition to the regular hotels. It is expected the government will continue, now that controls have been lifted, to hold down price increases for the paradores in order to set an example for hotels.

Even though prices at paradores will go up, they will still be a European bargain when one considers location, scenery, ambience, history, beauty and the design and craftsmanship that went into their construction. Add to that the fact that each has its own topquality restaurant, with local dishes and local wines featured.

Before deregulation, parador prices ranged from a low of about $15 for a double room in a three-star edifice to a high of about $30 in a four-star one, depending on the time of the year.

The Spanish parador system is so unique it deserves to be the subject of a guidebook. Until this year, sadly, there was none. Visitors had to depend upon skimpy information in tourism ministry brochures or occasional references in traditional guidebooks.

Now there is "Paradores of Spain," a book put out by a retired couple who spent five months visiting all the paradores. The writing in Sam and Jane Ballard's paperback guide is somewhat amateurish and the descriptions of the paradores were organized according to unofficial geographic areas that hardly anyone except Spaniards would be knowledgeable about. Otherwise, we found the book a valuable guide to our own recent parador tour in Spain.

Liberally sprinkled with color photographs of most of the paradores, the guide includes a historical description of each. It's available for $6.95, postpaid, from: Ballard's Travel Guide, 444 Washington St., Marina Del Rey, Calif., 90291.