At a time when a Holiday Inn has risen in the sleepy Peruvian town of Iquitos, when Master Charge is acceptable in the hill stations of Malaysia and martinis are served in Odessa, is there anything left for the adventuresome explorer?

Well, yes. If you're a little crazy and as unflappable as an ox, you can try seeing Brazil by bus.

On the grounds that you don't take in much of the country when you merely fly over it in a jet, busing through Brazil has appeal. There are, however, several large flies in this ointment.

For one thing, the maps you see all lie. Once there, you quickly learn that the real Brazil is at least four times larger. This is a country with an island that by itself is bigger than Belgium; flying from North to South takes about twice as long it takes from New york to Miami. And there are many unexplored stretches and unmarked boundary lines.

So never mind. To "see" an entire country is only a figurative expression in any case. I knew that. What I didn't know - and couldn't find out in the United States - was how Brazil's bus system operates, where it operates and when. That's probably because there is no one system. What does exist are some exceptionally good value buses, and I took one, finally, from Rio de Janeiro to Sao Paulo.

The trip started smoothly. There was a blur of people at the Rio terminal but they moved at a cheery, "just-looking" pace rather than meeting in shoulder-to-shoulder combat as they do in most metropolitan transportation terminals.

Even the man at the Informaciones desk smoothed my path. He answered my question ("Cometa Line?") with his won question ("Ticket?") and when i noded yes, pointed upstairs. There a ticket seller behind a sign reading "Sao Paulo 12-30" spotted me, waved a ticket and indicated I should move faster. It was, I saw, 12-25. I paid rapidly, then followed his sign-language directions to the bus. A handler stashed my suitcase in the baggage compartment and gave me a claim check. Minutes later we were off.

For the most pat, the scenery between Rio and Sao Paulo is high drama, Colorado with palm trees. A swifly moving river ran beside us awhile, and the hilly terrain made the broad asphalt highway look like a triumph of engineering over nature.

About midway in our six-hour ride, there was a rest break at a sprawling restaurant-gas station-gift shop complex that resembled a Howard Johnson's liberated by good-food addicts. For some reason, everything tasted like what it was supposed to be. On the list were fresh tropical fruit juices, rich ice creams and flaky pastries, plus a 60-cent ahham sandwich guarenteed to boggle the North American mind.

The bus, too, was well-equipped, with 34 well-padded seats, each slightly larger than its airline counterpart and capable of reclining to an even greater degree. There was a lavatory at the rear and curtains at all windows. Moreover, the price was right: less than $6 for the trip.

Similar bus-riding possibilities exist between most of Brazil's largest cities (with some major exceptions in the underdeveloped central and Amazon areas), many with several departures daily. Between Brasilia, the working capital, and Rio, the fun capital, there are at least half a dozen.

There are no giant, go-everywhere lines like Greyhound. A name to remember, though, is Iapermarin, Brazil's largest and most luxurious line, which operates in seven states in the southern and eastern parts of the country. Usually there are two competing firms operating between each city pair, but heaven help you when you go to find out about them. In Belem they can tell you about buses to Fortaleza south. In Brasilia they have the news on buses to Rio but not on buses from Rio to Recife.

Thiis means, of course, that it's difficult if not impossible to arrange space or be ticketed beyond your starting point. If you can't learn what's moving when, you obviously can't plot a program that isures onward connections. The nearest I could come to any schedule news was "The South American Handbook," as essential work even for area travelers.

The most important thing to know is that all services are "direct" between the two named points, meaning stopovers are out because there's unlikely to be alternative service between "minor" cities and no reboarding is allowed on the "through" service. At first this many no t sound significant but if you think about distances, it could give you the shakes. Once you buy a ticket for, say, Brasilia Rio, that means you travel the full 19 or so hours - Or Else.

Some leitos come with air-conditioning, some without. All reportedly include snack service and such amenities as carpeting, coverlets and toilets.

My Rio-Sao Paulo bus was "regular". In addition there's something called "executive" service in a few places. It features seats for 24-30 passengers, a lavatory and snack service. Regular service costs approximately 3 cents a mile, leito and excutive about double. All services have rest stops about every 2 1/2 to three hours, lengthened into meal stops at normal meal times. After dinner the main lights go out, though reading lights are permitted.

Industry people say the most scenic routes are between Rio and Recife and Rio and Porto Alegre. For the limited time that most U.S. visitors have, however, it's practical to try only one. I'd say follow the Rio-Recife route and try to break it up, concentrating on the colorful spots in the state of Bahia.

Bahia is both instant Africa and romanctic Colonial. It's also got the best shoppping in arts and crafts, an oversupply of white sand beaches and, some say, an even crazier carnival than Rio's. Consider the weather before you go, however. In their summer, a soff-centered America is apt to dissolve into a puddle of perspiration.