MADRID--Spain's Repsol SA said Thursday its net profit slid 5.7% in the first quarter, as tumbling crude prices weighed on its oil and gas producing unit, offset by a sharp increase in income at its refineries.

The Madrid oil and gas company reported a net profit of 761 million euros ($863.5 million) between January and March, compared with EUR807 million a year earlier. The figure for 2014's first quarter also included income from businesses that Repsol has since sold, including its liquid natural gas assets.

Adjusted net profit, which strips out the effect of changes in the accounting value of oil inventories as well as other extraordinary charges and gains, jumped to EUR928 million from EUR532 million.

Lower oil prices and high expenses for drilling took a heavy toll on Repsol's oil pumping and drilling division, leading the unit to report a EUR190 million loss in the quarter, compared with a EUR255 million profit a year earlier.

Offsetting this was an 84% increase in profit at the unit that includes Repsol's refining and chemicals operations to EUR534 million. The refining business makes more money when energy costs fall and in the first quarter its profit margins reached a record high.

Write to Christopher Bjork at christopher.bjork@wsj.com

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