NS&T Bank, the fifth-largest Washington bank, yesterday announced profit increases of 23.4 percent for the fourth quarter and 13.3 percent for the year.

Also reporting improved earnings yesterday were W. Bell & Co. Inc., Union Trust Bancorp, James Madison Ltd. and Computer Data Systems Inc., but CACI said that its earnings declined.

NS&T's fourth-quarter earnings climbed to $1.552 million ($1.13 a share) from $1.258 million (92 cents) during the same period last year. Profits for the year rose to $6.250 million ($4.55) from $5.518 million ($4.02) in 1983.

NS&T attributed the increase in earnings to higher loan volume and lower nonperforming assets.

* W. Bell & Co. Inc., a Rockville-based catalogue-showroom retailer, said its second-quarter profits for the period ended Dec. 29 climbed 10.4 percent to $2.118 million (70 cents a share) on sales of $60.052 million from $1.919 million (66 cents) during the same period last year on sales of $57,060 million.

Six-month profits rose to $2.264 million (75 cents) on sales of $87.293 million from $2.010 million (76 cents) a year earlier on sales of $81.805 million.

* Baltimore's Union Trust Bancorp, parent of Union Trust Bank, announced a 31 percent increase in profits for the fourth quarter and a 34 percent rise for all of 1984. Fourth-quarter earnings for the quarter totaled $4.8 million ($1.95 a share) versus $3.7 million ($1.50) for the previous year. For the 12 months, earnings were $19.2 million ($7.82), compared with $14.4 million ($5.85) in 1983.

Chairman J. Stevenson Peck attributed the record profits to an increase in volume and a widening spread between money costs and loan yields.

* Profits of CACI, an Arlington-based international professional and high-technology services company, said profits tumbled 49 percent in the quarter ended Dec. 31 to $658,377 (7 cents a share), from $1.3 million (14 cents) during the same period in 1983. Revenue was $24.6 million,compared with $30.2 million a year ago.

A company spokesman said the reduction in revenue resulted from a streamlining of unprofitable European activities, lower foreign currency rates and the winding down of a U.S. Navy contract in Saudi Arabia. During the same quarter, the company signed a five-year, $22 million contract with the U.S. Air Force for integrated logistic support at its consolidated space operations center.

* James Madison Ltd., the holding company for Madison National Bank of Washington, earned $962,742 ($1.16 a share) in the fourth quarter, up 22 percent from $788,709 (95 cents) a year earlier. The company experienced record earnings for all of 1984 as net income rose 21 percent to $2.8 million ($3.41) from $2.3 million ($2.82) in 1983. Loans increased by 27 percent; deposits, by 16 percent.

* Computer Data Systems Inc. of Rockville, a professional and processing services firm, yesterday said that for the six months ended Dec. 31, 1984 the professional and processing services firm increased revenue by 13 percent over the same period last year.

Computer Data reported revenue of $29 million, up from last year's revenue of $25.8 million. Net income was $1.6 million (54 cents a share) for the six-month period, up from $1.5 million (48 cents) last year.