The effect of human calcitonin (hCT) on the secretion of luteinizing hormone (LH) was examined in castrated rats. Male rats were orchiectomized (Orch) 2 weeks before they were treated with a single bolus injection of gonadotropin-releasing hormone (GnRH, 1 microgram/kg of body weight), hCT (3.4 ng/kg), or the combination of GnRH and hCT through a right jugular vein catheter. Blood samples (0.5 ml each time) were collected immediately before (0-min) and then at 15, 30, 60, 120 and 180 min after hormone injection. Plasma LH concentration was measured with a radioimmunoassay technique. Plasma calcium concentration was determined in an automatic analyzer. hCT significantly diminished both the basal and GnRH-stimulated levels of plasma LH without any relation to the reduced plasma levels of calcium. These results clearly demonstrated a plasma calcium-unrelated inhibitory effect of hCT on LH secretion.