A set of indoleamine accumulating neurons has been described in the retina of several species. Their transmitter is unknown though 5-hydroxytryptamine has been suggested. We have studied the 5-hydroxytryptamine concentration in the retina of cows, pigs, guinea-pigs and also of rabbits in which the blood had been washed out. It was found to be 26·2 ng/g ( s.e.m.=7·17) in cow retina, 5·9 ( s.e.m.=0·83) in pig retina and 6·03 ng/g ( s.e.m.=2·61) in guinea-pig retina. Concentrations in this range could well be due to blood platelet 5-hydroxytryptamine and this was corroborated by the about six-fold fall from the normal concentration to 3·8 ng/g ( s.e.m.=0·9) which occurred when rabbit retinas were perfused to remove the blood. The possible presence of indoleamine accumulating neurons in some mammals (cow and pig) was also investigated by means of fluorescence microscopy. No indoleamine accumulating neurons could be found with this technique. Nevertheless, autoradiography has shown that there are indoleamine accumulating neurons in the cow and pig retina, and we suggest that these neurons have the ability to modify the indoleamines they accumulate into substances related to 5-hydroxytryptamine but unable to form fluorophores with formaldehyde in tissues.