The newborn of 4 pregnant female rats were collected within 24 h of birth. Twelve neonates were used for the study of chromaffín reaction, by both the direct and the indirect methods. The medullary cells could be demonstrated at this stage; however, the demarcation between noradrenaline and adrenaline cells was not quite sharp. Application of formaldehyde-induced fluorescence to the newborn rat adrenal gland was highly significant. A clear distinction could be demonstrated between noradrenaline and adrenaline cells, the former giving off strong, white-green fluorescence and the latter being weakly green-fluorescent. A third type giving off moderate green fluorescence was interposed between the cells and possibly represented an equivocal cell type at this stage. Sympathetic nerves could be demonstrated in relation to blood vessels and medullary cells