Endocrinologists have conducted many studies on the effects of gonadal steroids and gonadotropin releasing hormone (GnRH) agonists with testosterone (T) as potential male contraceptives. Results of most of the studies show that treatments suppressed sperm counts but in >60% of subjects the levels did not fall to azoospermic levels. Research indicates that steroid treatments are more likely to achieve azoospermia in men from Indonesia and China than in Caucasian men. Research for stronger inhibitors of gonadotropin secretion began during the 1980s with GnRH antagonist analogs. Endocrinologists in California injected 8 men with 7.5 mg/day of the potent GnRH antagonist (Nal-Glu GnRH) and testosterone enanthate (TE2) every 2 weeks starting with the 2nd week for 16 weeks. 7 men experienced azoospermia within 6-10 weeks and LH levels fell below detectable levels. FSH also fell. TE2 levels fell to castrate levels by 2 weeks but returned to normal after the physicians administered TE2. TE2 administration preserved libido and sexual potency. LH suppression was not complete in the other man who withdrew during the 14th week because of considerable local swelling at the Nal-Glu injection site. At week 14 his sperm count measured 7 million/cc (42 million/cc during control). After treatment sperm counts and hormone levels of the 7 men reverted back to normal. These findings were better than other studies on Caucasians in which physicians administered only a high dose of T or GnRH agonists with T. These data and another studys data indicate that reversible azoospermia can occur in almost all men. Additional research is needed to improve the strength of GnRH antagonists which will result in lower costs and less complex T depot formulations. Current studies are examining whether severe oligospermia is sufficient to prevent pregnancy.