Atlantic Richfield Co. and Unocal Corp. posted sharply lower first-quarter profits yesterday, and Arco warned that its earnings "could deteriorate significantly" in the second period because of low oil prices.

Both companies blamed the setbacks on plunging crude-oil prices, while Unocal also cited high interest costs related to its costly stock buyback as part of its success in fending off corporate raider T. Boone Pickens Jr. last year.

Arco is the nation's sixth-largest oil concern. Unocal, parent of Union Oil Co. of California, is the ninth-largest.

Among other corporate reports released yesterday, Boeing Co. and Textron Inc. said their earnings rose, General Dynamics Corp. said its earnings were lower than a year ago and Morgan Stanley Group Inc. said its first quarterly financial results made public showed first-quarter profits were more than triple those for a year earlier.

Arco's first-quarter net income dropped 18.2 percent to $288 million ($1.58 a share) from $352 million ($1.47) in the first quarter of 1985. Revenue fell by 25.6 percent, to $4.29 billion from $5.77 billion.

Unocal said its first-quarter net income was $70.2 million (60 cents a share), a decline of 61.2 percent from the year-earlier period, when it netted $180.7 million ($1.04) on more shares outstanding. Revenue declined by 18 percent, to $2.28 billion from $2.78 billion.

Unocal said $103 million, or 93 percent, of its $110.5 million decline in profits was caused by lower crude-oil prices and financing costs for a $4.8 billion stock repurchase initiated to defend against unwanted takeover bids.

*Aircraft-manufacturing giant Boeing Co. yesterday reported first-quarter earnings of $148 million (95 cents a share) compared with its 1985 first-quarter earnings of $110 million (75 cents). Sales increased to $3.52 billion from $2.93 billion.

Earnings before federal income taxes were $233 million for the first quarter of 1986 compared with $165 million for the 1985 quarter. The increase came primarily from a higher sales volume and increased interest income, the company said.

*General Dynamics Corp. reported first-quarter earnings of $70.1 million ($1.64 a share) compared with $83.6 million ($1.98) for the same period in 1985. Sales increased to $2.1 billion from $1.9 billion last year.

The corporation said the reduced earnings were "wholly accounted for by the recently acquired business of Cessna Aircraft Co."

"The first quarter of 1986 saw continuing strong performance by three of our major defense business segments" -- military aircraft, missiles and gun systems, and submarine operations -- Chairman Stanley C. Pace said.

*Textron Inc., an aerospace and financial services concern, said yesterday that its first-quarter earnings rose $7 million to $49.5 million ($1.21 a share) from $42.5 million ($1.18) in the first quarter of 1985. Revenue was $1.5 billion compared with $1.3 billion.

Company officials said net income was $50.4 million ($1.23), down $10 million from the first quarter of 1985, largely because a one-time accounting change added $11.8 million to the 1985 figure.

Textron President B. F. Dolan attributed the overall improvement to strong performances in the turbine engine and industrial products division of the firm and gains from the sales of securities.

*Morgan Stanley Group Inc. said the strong stock and bond markets helped its first-quarter net income increase to $58 million ($2.92 a share) from $17.8 million (89 cents) a year earlier. Revenue climbed 51 percent to $589.6 million from $389.5 million, while expenses rose 35 percent to $486 million from $358.7 million.