Commission rates for small investors will be increased slightly at some brokerage firms next week, but no industrywide boost appears imminent.

Smith Barney, Harris Upham & Co. is raising commissions on retail trading by 2.8 percent on Monday while Bache Halsey Stuart Shields Inc. will raise rates for its individual customer trades by up to 1 1/2 percent, effective next Thursday.

Other large national brokerages as well as regional firms in the Washington area, which dominate the retail or individual investor trading marketplace, said they have no immediate plans to change rate structures.

Competitive pricing in brokerage rates began on May 1, 1975 -- called "May Day" in the securities industry -- when the historic practice of fixed rates was abolished. Since then, rates for smaller investors generally have risen and charges to large institutions that trade heavy volumes have decreased as brokers battled for their business.

At most firms, retail rates jumped some 10 percent initially and, a year ago, by an additional 5-10 percent. At the same time, a few "discount" brokers have started to charge sharply lower retail commission for individual investors in exchange for the basic service of stock trades.

Alex Heckman, manager of Bache's downtown Washington office, said his firm's "very slight" increases next week are designed to "even out" rates on some types od trades where Bache has been "under the market" -- charging less than the competition.

Heckman also pointed out that the nature of brokerage firm business has changed in recent years from one where there was active trading of stocks to one that is "income-oriented," with customers most interested in long-term investments.

Before fixed rates were ended, a brokerage firm was charging $71.50 for a $5,000 trade of 100 shares of $50 stock. Today, most brokerage firms charge more than $80 for such a trade. Smith Barney's new rates on this $5,000 trade will be $85 (rounded down from $85.34), compared with $82.05 currently.

At Alex. Brown & Sons, the same trade would result in a commission of $81.73, according to Norman Farquhar, the firm's partner who heads Brown's office here. He said there have been "no discussions" recently about changing rates.

Similarly, Johnston, Lemon & Co. President James Lemon said "nothing is planned," although "we constantly are looking at" the rate situation. Both Alex. Brown and Johnston, Lemon raised rates a year ago -- Johnston, Lemon by 10 percent.

At Kulak, Voss & Co., a discount broker in Northern Virginia, no rate changes are planned although the minimum commission was increased to $30 from $25, six months ago. On the same $5,000 trade cited above, Kulak, Voss charges $53.63 for a one-time commission and as little as $35.73 for regular customers who trade often.

Johnston, Lemon, meantime, reported that for the second consecutive year an index of local company stocks outperformed standard market measurements. The Washington investment firm follows stocks of 30 area corporations -- the index of which rose nearly 13 percent in 1978 to 90.783. The Dow Jones average of 30 New York Stock Exchange industrials fell 3.1 percent last year, the Standard & Poor's index of 500 NYSE stocks rose 1.1 percent and the NYSE's own composite index advanced 2.1 percent.

Of the 30 stocks in the daily local index, 17 were up an average of 26.4 percent while 12 were down an average of 11.8 percent and one was unchanged for the year.