The regulation of post-mating decline of sex-pheromone in the gypsy moth, Lymantria dispar, was studied. An initial, transient suppression of pheromone production was found to be caused by the introduction of male genitalia into the bursa copulatrix, which results in mechanical pressure being transmitted via innervation of the bursa. However, if sperm was not transferred during mating, pheromone production resumed and females returned to calling behavior. Permanent suppression of pheromone production resulted from an adequate supply of sperm in the spermatheca and could be prevented in females from which spermatheca was removed. During the initial period of suppression of pheromone production females were sexually receptive and could remate. They became nonreceptive only when pheromone production was terminated and oviposition begun.