HOUSTON (AP) _ Tenneco Inc. reported a net income of $124 million, or 73 cents a share, for the first quarter of 1986, compared with a net loss of $54 million for the same period last year, company officials said.

The results reflect a change, during the quarter, in the company's method of accounting for oil and gas operations, from the ''full-cost'' method to the ''successful efforts'' method. The decision was made in light of deteriorating oil and gas prices, officials said.

Full-cost accounting permits a company to spread out the gains and losses of all its exploration costs instead of writing them off against present earnings.

Under the successful efforts method, companies charge off unsuccessful exploratory efforts against current earnings, while capitalizing only successful wells.

Without the accounting change, Tenneco's earnings would have been about $104 million, or 60 cents a share, officials said. The cumulative effect was to reduce prior years' earnings and retained earnings, as of Dec. 31, 1985, by $988 million, they said.

Originally, net income for the first quarter of 1985 was reported as $63 million, or 34 cents per share. When considered in light of the accounting change, the same period's results work out to a loss of $54 million on revenue of $3.66 billion.

The company also reported revenue for the first three months of the year at $3.95 billion.

Tenneco is the nation's ninth-largest oil company. Its total revenues last year were $15.3 billion.

''Tenneco, like others in the oil and gas industry, has been negatively impacted by the recent serious decline in energy prices,'' said James L. Ketelsen, chairman and chief executive officer, on Monday. ''Our diversification, however, has enabled us to more than offset these lower energy prices with strong performances in a number of our non-energy operations.''

The company also reported that the operating income from natural gas pipelines declined due to decreased sales and transporation margins and another charge for employee severance costs.

Ketelsen said although there are uncertainties about future oil and gas prices, Tenneco has taken a number of steps to reduce its costs to ensure that it will remain a strong competitor in the energy business.

Tenneco's non-energy divisions including construction and farm equipment, insurance and agriculture, and land management had much improved operating results for the quarter.

''The company's diversification continues to provide a broad base of profitable business opportunities despite the difficulties in the energy sector, and we have made the necessary adjustments to perform well in the current environment,'' Ketelsen said.