E.I. du Pont de Nemours & Co. yesterday reported declines of 20 percent in both sales and earnings for the first quarter.

The results included those of Conoco, the oil and coal firm acquired last year.

Net income for the quarter was $250 million ($1.06 a share) on sales of $8.3 billion compared with $314 million ($1.33) on sales on $8.1 billion pro forma a year ago and with $208 million ($1.32) on sales of $3.7 billion for Du Pont alone a year ago.

Chairman Edward G. Jefferson said the acquisition of Conoco increased this year's first-quarter profits by about 30 cents a share.

He said lower prices cut petroleum profits, while petroleum marketing, refining and transportation earnings were up 59 percent. Coal earnings were more than double those of a year ago. Agricultural and industrial chemical profits were down 5 percent despite an 11 percent sales gain. Profits from polymer products fell to $10 million from $29 million, and profits on fiber products were down 61 percent. Medical-product profits were higher, but general industrial and consumer products earnings were off 10 percent.

Greyhound Corp.'s first-quarter profits fell 40 percent to $14.8 million (33 cents a share) from $32.9 million (74 cents) as revenues slipped to $1.03 billion from $1.09 billion.

First-quarter profits last year included a one-time gain of $8.1 million from settlement of litigation with International Business Machines Corp.

Chief Executive John W. Teets said the decline in operations was due primarily to softness in its transportation group, which was hurt by poor weather in January and February and by the recession.

Republic Airlines Inc. had a $22.6 million loss in the first quarter compared with a loss of $15.2 million a year earlier as revenues rose by 6 percent to $372.7 million from $351.1 million.

President Daniel F. May said Republic's loss was one of the smallest in the industry.