Acyline is a novel GnRH antagonist found in animal studies to be a potent suppressor of circulating gonadotropin and testosterone (T) levels. We conducted the first study of acyline administration to humans. Eight healthy, eugonadal young men were administered a series of acyline injections (0, 2.5, 7.5, 25, and 75 μg/kg), each injection separated by at least 10 d. Serum FSH, LH, and T levels were measured for 7 d after injections. Acyline suppressed FSH, LH, and T levels in a dose-dependent fashion. Maximal suppression occurred after injection of 75 μg/kg acyline, which suppressed FSH to 46.9 ± 2.5%, LH to 12.4 ± 2.2%, and T to 13.4 ± 1.4% of baseline levels, maintaining suppression for over 48 h. Serum acyline levels peaked at 1 h at 18.9 ± 0.9 ng/ml, remained significantly elevated above background 7 d after injection, and returned to background levels by 14–17 d after injection. Side-effects at the site of injection were limited to infrequent blush and pruritus that resolved within 90 min of injection. Higher doses of acyline might be effective as depot injections for long-lasting gonadotropin suppression in hormone-dependent diseases and for use in male hormonal contraception regimens.