The effects of coitus on the ultrastructure of neurons in the preoptic nucleus of the rabbit were studied. The ultrastructural changes observed first became apparent in large neurons located near capillaries, 1 h after coitus. More pronounced changes were observed in neurons removed at 2 h through to 10 h after coitus. These changes were manifest primarily in cell organelles normally associated with synthesis, in particular the Golgi system and rough endoplasmic reticulum (ER). Two populations of dense-core granules are reported. The small Granules (mean diameter of 950 Å) are probably monoamine in nature; the larger granules (mean diameter 1,450 Å) may contain releasing factors or their carriers. There is no evidence of storage of secreted materials within the perikarya of these neurons. Immediate transport toward the median eminence is suggested.