Ford Motor Co. posted a $728 million profit for the year's first quarter, a 7 percent decline from the same quarter last year.

Yesterday's report from the nation's second-largest car maker completed Big Three results for the Jan. 1-March 31 period showing that while profits were down from last year, 1986 will be a fourth consecutive healthy year for Detroit.

E. I. du Pont de Nemours & Co. said its net income for the first quarter was up from the same period last year.

Among oil companies, Texaco Inc. said that its first-quarter earnings grew by 3 percent; Standard Oil Co. and Pennzoil Co. said their earnings fell by 26 percent, and Amerada Hess Corp. reported a loss.

Ford said its consolidated profit came to $2.70 a share compared with $783 million ($2.80) a year earlier. Without adjusting for the recent three-for-two stock split, the per-share earnings would have been $4.05 and $4.20 respectively, Ford said.

General Motors Corp. earlier listed a 1 percent decline in profit at $1.06 billion this year vs. last year and Chrysler Corp.'s income was off 30 percent at $357 million.

First Nationwide Financial Corp., in its first full quarter as a Ford subsidiary after its $500 million purchase last year, earned $22 million, Ford said.

*Du Pont Co. said its first quarter net income was $404 million ($1.67 a share) compared with $143 million or (59 cents) for the same quarter last year.

After-tax operating income for the Wilmington, Del., company's chemical and specialty products businesses was $253 million, or $38 million ahead of the same quarter last year.

After-tax operating income for the energy businesses was $195 million, or $53 million above last year's first quarter.

*Texaco's income was $328 million ($1.37 per share) up from $320 million ($1.28) in the first quarter of 1985, the company said from White Plains, N.Y.

*Standard Oil, formerly known as Standard Oil Co. (Ohio), or Sohio, reported $253 million ($1.08 a share) in net earnings, vs. $343 million ($1.46) in the 1985 first quarter. The company changed its name in February.

*Pennzoil officials said the Houston company earned $42.3 million (93 cents a share) in the period between January and March, compared with $57.4 million ($1.20) in the year-ago period.

*Amerada Hess, which is based in New York, said it lost $339.3 million in the first quarter, as opposed to a year-ago gain of $37.2 million.

*Johnson & Johnson, beset by a second round of Tylenol poisonings, announced a $136.5 million first-quarter loss, its first quarterly loss since going public 42 years ago.

Bolstered by the weaker dollar, the Brunswick, N.J., company's sales for the quarter increased 9.7 percent to $1.74 million from $1.59 billion in the first quarter of 1985, Chairman James E. Burke said.

*Allied-Signal Inc. had record net income of $186 million for the first quarter of 1986, a 39 percent increase over the $134 million earned in the same period last year, the company said.

Earnings per share of 98 cents for the first quarter were down 28 percent from last year's $1.37 because the New Jersey company more than doubled its number of outstanding shares to 176 million, as a result of the September 1985 acquisition of the Signal Cos. Inc., said Chairman Edward L. Hennessy Jr.

*Philip Morris Cos. Inc.'s first-quarter net earnings jumped 23.3 percent and operating revenue soared 78.7 percent, the tobacco and food products conglomerate reported.

Net earnings for the three months ended March 31 totaled $316 million ($1.32 a share) compared with $256 million ($1.06) in the comparable 1985 period, the New York company said in a statement.

*Clorox Co. reported net third-quarter earnings were $26.4 million (99 cents a share) an 8.6 percent improvement over $24.3 million (92 cents) earned during the same period a year ago.

Net sales for the quarter were up 4.5 percent to $279.6 million from $267.5 million the previous year.

*Walt Disney Co. reported a 48 percent increase in first-quarter 1986 earnings of $51.8 million (38 cents a share) compared with $34.9 million (26 cents) during the same period last year.

The entertainment company also reported a 29 percent earnings increase to $86.5 million (64 cents a share) for the six months ending March 31, compared with $67.1 million (50 cents) for the same period the previous year.