Studies were performed to examine the mechanism by which testicular Leydig cell tumors are induced in rats by administration of the antimicrobial agent oxolinic acid (1-ethyl-1,4-dihydro-6,7-methylenedioxy-4-oxo-3-quinolinecarboxylic acid). In these studies, the effects of oxolinic acid on serum levels of luteinizing hormone (LH), testosterone, and prolactin and the binding of testosterone to prostatic androgen receptors were examined. In a long-term hormonal study, male Wistar rats were fed a diet containing oxolinic acid at 0, 100, 1000, or 3000 ppm for 104 weeks. A statistically significant increase in serum LH levels was observed at 1000 and 3000 ppm, but no dose of oxolinic acid had a significant effect on serum testosterone levels. Serum LH levels were no longer elevated above control levels within 2 weeks of cessation of the administration of oxolinic acid. Oxolinic acid was found to have no effect on the rate of clearance of exogenous LH from the circulation. Serum prolactin levels were decreased by the administration of oxolinic acid. The increase in serum LH induced by oxolinic acid was completely blocked by the intraperitoneal injection of the dopamine antagonist haloperidol (2 mg/kg). In addition, no significant affinity of oxolinic acid for androgen receptors was found in an in vitro study. These findings suggest that: (1) oxolinic acid induces Leydig cell tumors in rats by chronically stimulating the release of LH from the pituitary, (2) the mechanism of stimulating the release of LH involves facilitation of the dopaminergic systems in the hypothalamus-pituitary axis, and (3) oxolinic acid has no effect on androgen-mediated feedback inhibition.