Pfizer Inc. said Wednesday it paid about 4,500 doctors and hospitals $35 million during the second half of last year to study how the company's medicines work and to promote the treatments, in its first public disclosure of payments to the professionals and institutions that test and prescribe its products.

Drug companies have been under pressure from some members of Congress and physicians to reveal their payments. The health overhaul passed recently requires disclosures of certain physician payments starting in 2013. But...