Implants of cocoa butter (CB) or CB and actinomycin D (AD) were placed in the hypothalamus, amygdala, or anterior pituitary gland of testosterone-pretreated ovariectomized rats. The animals were then given an injection of 2.5 mg progesterone (P) at 11.00 h and sacrificed at approximately 16.30 h. Serum LH was determined by radio-immunoassay. Implants of AD placed in or near the arcuate nucleus and in other sites in the ventromedial hypothalamus blocked P-induced LH release, but those placed in the preoptic area or ventrolateral hypothalamus were only partially effective. Implants of AD in the amygdala or anterior pituitary gland had no detectable effect on LH release. Approximately 70% of the radioactivity in implants of 3H-AD diffused from the implant site within 8 h. Of that recovered, most was found within 1 mm of the implant site. These results suggest that the ventromedial hypothalamus is one site at which AD acts to block P-induced LH release.