Ovariectomized female rabbits were pretreated with actinomycin D (80 µg/100 g body weight) cycloheximide (300 µg/100g body weight) or chloramphenicol (300 µg/l00 g body weight) 4 h before an intravenous injection of 25 µg of estradiol benzoate. Actinomycin D, cycloheximide and chloramphenicol decreased basal gonadotropin levels, but actinomycin D was less active than chloramphenicol and cycloheximide. Despite this effect on basal levels, such treatment did not prevent the effect of estradiol benzoate on LH release. Moreover, no significant impairment of the LH response to LH-RH was found. The interpretation of experiments using drugs such actinomycin D, cycloheximide or chloramphenicol requires caution; however, one cannot disregard the possibility that the negative feed-back effect of estrogen, in contrast to the positive effect, may not be mediated by RNA-dependent DNA and the protein-synthesizing systems.