Catecholamines were found histochemically in the visceral yolk sac of the rat from embryonic day (ED) 10, i.e. before the amines become detectable in peripheral or central neurons of the fetus. Formaldehyde-induced fluorescence was confined to the apical part of the yolk sac epithelial cells. The specificity of histofluorescence has been confirmed by borohydride reduction, microspectrofluorimetry revealing an emission peak at 480 nm and administration of reserpine. The catecholamines present were identified by mass fragmentography using N,O-trifluoroacetyl derivatives. At ED 13 both dopamine and norepinephrine were present, while only dopamine was detected at ED 18 1/2. Maternal circulation or the epithelial cells themselves appear as possible sources of these catecholamines. The occurrence of amines in the yolk sac epithelium may reflect an intracellular role of these compounds, a barrier function of the epithelium or a step in a transport to the fetus where the amines might assume regulatory functions.