Cytochemical studies of adrenomedullary hormones in 3 species of ophidians reveal that the adrenal gland produces both adrenaline and noradrenaline, but noradrenaline chiefly by the dorsal chromaffin band and adrenaline by the intercortical chromaffin islet. Adrenomedullary hormonal zonation was found in the viper only. Cytochemical and biochemical analyses also reveal that adrenaline and noradrenaline are produced in equal proportion only in the krait (dual secretor) but in different proportion in the viper (chiefly adrenaline secretor) and in the cobra (chiefly noradrenaline secretor). Hormonal content was highest in the krait, moderate in the viper and lowest in the cobra. Wide ranges of variation in the proportion and total concentration of these hormones have been suggested to be related with behaviour and/or species specificity.