Twenty-six men with severe oligospermia (sperm density less than 10 X 10(6)/ml, but greater than O), but normal serum concentration of FSH, as well as normal serum concentrations of LH and testosterone, were given a 250 microgram i.v. bolus dose of synthetic gonadotrophin releasing hormone (LHRH). The serum FSH and LH responses were compared to those of a group of normal men and a group of men with oligospermia or azoospermia and elevated basal serum FSH concentration. The mean FSH response to LHRH of the men with oligospermia but normal basal serum FSH concentration was 911 miu min/ml, nearly three times that of the normal men, 322 miu min/ml (P less than 0.001), though not so great as that of the men with oligo/azoospermia and elevated basal FSH concentration (2890 miu min/ml). Ten of the twenty-six men with oligospermia and normal basal serum FSH had a supranormal FSH response to LHRH. The mean LH response to LHRH of the men with oligospermia and normal basal serum FSH levels was not significantly different from that of the normal men. We conclude that LHRH stimulation can often elicit a deficiency of feedback inhibition of FSH secretion not readily recognizable by measurement of the basal serum FSH concentration.