The National Press Building Corp. says that it is lowering its rental rates in recognition of both the softness of the office rental market and the disruption that the $65 million renovation of the building has caused.

In a letter to tenants, corporation President William D. Hickman said Friday that news organizations moving into the first of the completed parts of the building will save $6 a square foot on the original price of $26 so long as there is any work in progress on their floors.

Tenants who move into the first and second completed parts of the building will have their rental rates marked down by $3 until the building is nearly complete, the letter said.

Tenants are expected to move into renovated space beginning in May 1983 and into the second part to be completed in January 1984.

The reductions in rental rates of between 11 and 23 percent are " a pretty big concession on our part, but the marketplace dictated it," Hickman said.

Vivian E. Vahlberg, corporation chairman, said the reduction also reflects the recognition "that we were asking tenants to endure a lot of disruption and discomfort and still charging them a lot of money."