An internal investigation by the Interior Department has disclosed that one-fifth of the equipment and other property belonging to the National Park Service in the Washington area is missing and cannot be found because of poorly kept records.

Among the $2.9 million in missing items, according to the investigative report, were a tractor and a trailer-mounted electrical generator, each valued at more than $8,000, and an assortment of other equipment, including five cameras, five radios, three refrigerators and four handguns.

The probe also found that the Park Service failed to keep track of up to $700,000 in automotive spare parts and maintenance equipment received and issued each year at its Brentwood storehouse in Northeast Washington, raising "serious concerns about the possibility of fraud, theft and abuse."

In addition, the investigation showed that park officials "displayed a complete disregard for government requirements" in certifying that other items were lost, damaged or worn out when, in fact, they did not know what happened to them or what shape they were in. Among items in this category were 125 lawn mowers and 165 .38 caliber revolvers, the latter purchased for use by Park Police officers and park rangers.

The investigation of the National Capital Region of the Park Service, an arm of the Interior Department that oversees and maintains the major monuments of Washington and a network of parks in three nearby states, was conducted by the Interior inspector general's office. Its findings were reported to the White House's Office of Management and Budget.

Overall, the picture portrayed by the 64-page report, which was dated Aug. 18, was one of faulty or slipshod record-keeping and not of corruption, leaving open the possibility that some of the items may actually be in use.

A spokeswoman for Manus J. (Jack) Fish, regional director of the Park Service, said "a lot of corrective actions" have been ordered, but "some of these may take a while" to go into effect.

The inspector general's report quoted Fish's office as saying there was "no intentional act . . . to violate established regulations, nor any direct effort to misuse the trust placed on the employes for the protection, conservation and economical use of government property."

The estimate that $2.9 million worth of equipment is missing came from a statistical sample of items listed on the inventory of durable property owned by the regional park agency, as distinct from maintenance supplies that are routinely consumed and replenished.

The inventory that was checked, which goes back to 1970, included 14,200 items that originally cost $14.6 million. Hence, if the statistical check is accurate, a staggering 20 percent of all the listed property is missing. The worst record among 16 park units, the report showed, was regional headquarters itself.

The report cited unusual difficulty in verifying the location of revolvers owned by the Park Police and used by officers and rangers. Of a sample of 12 guns issued by the police to rangers with law-enforcement duties, 10 listings were found to be wrong. "One park ranger had left the Park Service in 1978 and his law-enforcement equipment, including his revolver, could not be located," the report said. Two other rangers said that, contrary to the records, they never had been issued guns.

The inspector general also said that many types of items, such as garden equipment and sound amplifiers that would be attractive targets of thievery, are not carried on the books as durable items and are not adequately accounted for. It urged that procedures be tightened.

For example, among recently purchased items, the report found that three wheelbarrows, two pairs of snowshoes, two ceiling fans and a dehumidifier were missing.

Investigators reported they could not determine from available records what was on hand at the Brentwood facility, where auto parts, lumber, electrical supplies, plumbing supplies and carpentry items are kept.

Of 40 items checked, actual inventories were wrong on 37, either over or under the actual amount. There were shortages of 50 spark plugs and 75 automotive oil filters and a surplus of 91 boxes of toilet tissue, the report noted. Since then, the regional office conducted an inventory of items there, using outside personnel.