We examined the effects of elevated thyrotropin-releasing hormone (TRH) levels on the gonadotropin response to gonadotropin-releasing hormone (GnRH) in 16 healthy men. A TRH infusion, which was begun 15 minutes before and continued for 120 minutes after the bolus administration of GnRH, was compared with a saline infusion. During the TRH infusion the gonadotropin responses to acute GnRH stimulation were higher than in the saline control period but, although statistically significant, the differences were small in magnitude and were similar to the intraindividual coefficient of variation in gonadotropin responses determined under control conditions. They did not appear to be of clinical significance. TRH significantly lengthened the interval between GnRH administration and the appearance of peak gonadotropin levels. These results indicate that a brief TRH infusion, producing mild elevations of thyrotropin and prolactin, causes slight alterations predominantly in the timing of GnRH-stimulated gonadotropin secretion.