With the peroxidase-antiperoxidase (PAP) preembedding immunocytochemical method the luteinizing hormone-releasing hormone (LHRH) containing fibres in the mesencephalic central grey substance (MCG) of male and female rats were studied light and electron microscopically. Thin unmyelinated LHRH immunoreactive fibres were found in the MCG up to the ponto-mesencephalic junction in a ventral sub- and intraependymal position. Especially in the middle part of the MCG, LHRH reactive fibres were found in two or three rostrocaudally orientated ventromedial thickenings of the ependymal lining. Ultrastructurally, LHRH varicosities contained immunopositive secretory granules with a diameter of approximately 110 nm. Because the LHRH varicosities were not observed to make any synaptic contact with dendrites of MCG cells, it seems very likely that LHRH influences MCG neurons in a nonsynaptic way. No apparent differences were found between male and female rats.