The effect of medroxyprogesterone acetate on the urinary excretion of LH and FSH has been studied in 5 patients with isosexual precocity and 2 castrate adults, by specific bioassay and radioimmunoassay methods. The drug suppressed urinary LH and FSH excretion in 2 of the children with idiopathic precocious puberty and both adults. The effect was seen with both oral and intramuscular preparations and it occurred in both sexes. Two of the 3 children whose gonadotropin excretion was not suppressed by the drug had been diagnosed as having “idiopathic” precocious puberty. Thus, medroxyprogesterone acetate, in the doses tested, does not suppress gonadotropin excretion in all cases of idiopathic precocious puberty. Failure to respond to medroxyprogesterone acetate with suppression of gonadotropin excretion may indicate an abnormality in responsiveness of the pituitary-gonadal axis in patients with isosexual precocity, and may also serve as a useful guide to the potential efficacy of treatment.