Five male patients with infertility due to biopsy-proven germinal cell aplasia were given a 100-microng bolus of luteinizing hormone (LH)/follicle-stimulating hormone (FSH)-releasing hormone (LRH), and the resultant LH, FSH, and testosterone responses were correlated with their histologic patterns on testicular biopsy. The basal and stimulated FSH values were elevated in four of five patients. Basal LH values were elevated in three patients, while two clearly had exaggerated LH and testosterone responses to LRH. Although there was little correlation between various parameters, elevated basal LH values were associated with Leydig cell hyperplasia. Higher basal FSH levels were present when fibrosis and complete aplasia of germinal elements were found in the same biopsy specimen, and the magnitude of the FSH response to LRH correlated positively with the basal concentration. The findings of elevated basal LH values, an exaggerated LH response to LRH, lack of testosterone response, and Leydig cell hyperplasia indicate a definite disturbance of the LH-testosterone axis in many patients with germinal cell aplasia. Therefore, the regulation of secretion of both gonadotropins appears to be abnormal in this disorder.