Acute (5 min) graded electrical stimulation (biphasic pulses: amplitude, 50, 200, and 500 microA; frequency, 10 Hz; duration, 0.5 msec) of the medial preoptic area (including the medial and lateral septal nuclei and amygdala, pars medialis) induced significant elevations in circulating gonadotropins (more luteinizing hormone, LH, than follicle-stimulating hormone, FSH) in male bullfrogs, Rana catesbeiana. Pituitary response to all currents was rapid and of brief duration; levels of plasma gonadotropins (LH and FSH) peaked at 5 min (the end of stimulation) and decreased rapidly thereafter (LH by at least 50% after 30 min). A stimulus-response relationship was apparent; the magnitude of LH maxima was directly dependent on current intensity between 50 and 500 microA. In general, pituitary responses characteristic of acute electrical stimulation were similar to those induced by injection of mammalian gonadotropin-releasing hormone (GnRH). This is the first direct physiological demonstration of GnRH-like activity in the brain of the bullfrog.