The autonomic nerve supply of the guinea-pig ovary was investigated by a combination of light- and electron microscopy. At the light-microscopic level, adrenergic fibres were identified due to their formaldehyde-induced fluorescence. In addition, the ovary contained acetylcholinesterase-positive fibres. In all parts of the ovary, the adrenergic fibres were most numerous. At the ultrastructural level it was possible to identify the adrenergic nerve terminals with the aid of the false adrenergic transmitter, 5-hydroxy-dopamine. Thus, large numbers of adrenergic terminals, characterized by their content of 50-60 nm, electron-dense synaptic vesicles, were seen within the interstitial gland, where they formed close contacts with the endocrine cells (membrane-to-membrane distance, 20-100 nm). The follicular theca externa was also richly supplied by adrenergic nerves. At this location, close contacts (50-100 nm) were identified between the nerve terminals and the smooth muscle-like cells. Very few adrenergic nerve fibres were present in the theca interna of follicles or in the corpus luteum. Non-adrenergic nerve terminals, characterized by electron-lucent synaptic vesicles of 50-60 nm diameter, were observed together with the adrenergic fibres. They were always present in much lower numbers than the latter. No "p-type" nerves were identified by electron microscopy.