Strong sales of duplicators and related supplies helped Xerox Corp. increase profits 27 percent in the first quarter, the company reported yesterday.

Profits for the period ended March 31 were $133.8 million ($1.66 a share), comparable year-ago quarter. Revenues were $1.57 billion, up from $1.35 billion in the same period in 1978.

A Xerox spokesman said that profits before taxes and net income grew faster than revenues, mainly due to favorable foreign exchange conversions. Xerox also publishes educational materials.

Rental and service revenue increased 12 percent over the comparable 1978 quarter, while the sale of copiers and duplicators and other related material rose 27 percent.

Xerox said that its quarterly profits were the highest for any first quarter in the company's history and that the operating revenues in the first three months of 1979 were a record for any Xerox quarter.

Monsanto Co. announced first-quarter earnings of $161.8 million ($4.44 a share) on sales of $1.62 billion.

John W. Hanley, chairman and president, said the earnings figure represents an increase of 19 percent over the $135.7 million ($3.71) reported for the same period last year on sales of $1.34 billion.

"Monsanto's first-quarter results reflected strong export sales, an improvement in the results of our business units that operate outside of the United States and higher domestic sales volumes," Hanley said.

With sales running 30 percent ahead of 1978, Lockheed Corp. had net earnings of $12.4 million (77 cents a share) for the first quarter, up from $3.2 million (13 cents) in the same period last year.

Excluding the L-1011 TriStar program, profits before other income, interest and income taxes were $66.9 million this year compared to $66.9 million for the first quarter of 1978.

A loss of $35.9 million was recorded in the TriStar program during the first three months of this year, while a $29.7 million deficit was run up in the first quarter of 1978.

Three customers took delivery on TriStars in the past quarter compared with none the previous year, but Lockheed officials said the increased loss was due to higher than expected material and manpower costs on aircraft under production.

Sales for the quarter included $539 million to the U.S. government, $103 million to domestic commercial customers and $365 million to foreign commercial airlines and governments.

Missouri Pacific Corp. reported record financial results for the first quarter of 1979.

Downing B. Jenks, chairman of the board of the St. Louis-based company, said net income increased 37 percent to $31.8 million ($2.03 a share) over earnings of $23.2 million ($1.60) for the same period in 1978. Revenues increased to $506.2 million this year from $426.1 million in the year-earlier period.

Jenks said all of MoPac's operating companies contributed to the first-quarter gain. The firm operates the Missouri Pacific Railroad as well as a natural gas pipeline company and a cement company.

Consolidated Edison Co. had a drop in first-quarter profits to $1.12 a share from $1.41 a year ago caused by a state public utility commission order reducing the company's winter power rates.

The company said profits and sales were also slashed by lower purchases of Con Ed power by other utilities.

Net income tumbled to $80.28 million on revenues of $810.03 million from $97.92 million on revenues of $814.71 million.

American Broadcasting Cos. Inc. reported record earnings for the first quarter that are up 11.2 percent from the same period last year.

ABC earned $25.5 million (91 cents a share) in the latest quarter, up from $22.9 million (84 cents) in the first three months of 1978.

ABC became the only network to report increased profits from its television operations. Both CBS and RCA, which owns NBC, earlier reported declining broadcasting profits due to the high costs of new programming they are using in an effort to catch ABC in the ratings.

ABC revenues in the quarter rose 8.8 percent to $451.6 million from $415.1 million in the same quarter last year.

The company's 1978 quarterly earnings included $1.5 million, or 6 cents a share, on operations since discontinued.

ABC said earnings from radio operations declined slightly and said its leisure group lost money during the quarter, largely due to problems with the "historic towne of Smithville," whose sale the company announced April 10