A 12.5 percent prime interest rate, a 7.3 percent increase in inflation and "a mild recession" were forecast today by economists for T. Rowe Price Associates, the big Baltimore investment firm.

In their annual briefing on the economy, Price executives also predicted a 2 percent increase in the gross national product in 1979 and a 4 to 6 percent growth in corporate profits.

But taking a longer view, Rowe Price Chairman E. Kirkbride Miller said the outlook is for a "better investment enviroment in 1979" largely because of the depressed level of stock prices.

Despite the bargains, however, economists and investment specialists for the $60 billion gitm said they see no great migration of investors back to the stock market and a diffcult year ahead for bond buyers.

T. Rowe Price Manages about $4 billion in pension, profit sharing and endowment funds and runs mutual funds with assets of about $2 billion.

George Collins, the vice president in charge of fixed income investments, predicted the prime lending rate will raise to 12 percent after the first of tye year and peak at 12.5 percent late in the second quarter of 1979.

Collins said Price's projections are that interest rates will decline-to 10.5 or a 10.5 or 11 percent prime-later in the year. Running down other interest rates, he predicted yields of 9.5 percent of Treasury bills, 10 percent on utility bonds and 9.25 percent on industry bonds.

The likelihood that interest rates will decline later in the year will lead the firm to increase its investments in money market and bond obligations "at some point in 1979," added Edward Mathias, vice president for investment policy.

Acknowledging "the timing remains uncertain" Mathias said he was confident interest rates would peak at levels comparable to the historic levels reached in 1974."

Ben Laden, Price's chief economist, warned of "sluggish economic growth and persistent high inflation" in the coming year. "When we look back we'll call it a mild recession or growth recession," he said.

Predicting a 7.5 percent inflation rate and growth in real gross national product of just over 2 percent, Laden said the critical comparisions that indicate the changes in the economy are not the yearly averages, but between the final quarter of this year and next.

Real GNP is growing at a rate of 3.9 percent now, but will slow to a rate of .6 percent in the fourth quarter of 1979, he predicted.

Laden said he believes business investment will fall from the current 16.9 percent rate to 12 percent by the end of next year, and residential construction, which is up 10.8 percent this year, will be down 9.9 percent.

Business purchases of inventory are increasing at a rate of 4.9 percent this year, but will be cut 8 percent next year, he added.

Rowe Price executives would not guess what would happen to the Dow Jones Index or stock market averages in the next year; regardless of what happens, they said, there are no signs of a return of investors to the market.