Varying degrees of monoamine oxidase (MAO) activity were noticed all along the hypothalamo-hypophyseal neurosecretory tract in C. batrachus. Intense activity was evident on the wall of the secretory cells of the adenohypophysis, neurons of the nucleus preopticus (NPO) and nucleus lateralis tuberis (NLT). However, MAO activity was also observed in the cytoplasm of some neurons in the ventromedian region of the NLT and groups of cells present in the neurohypophyseal ramifications of the pars intermedia. A strong accumulation of formazan granules in the median eminence (ME) and the neuroadeno interface lends further support to the view that C. batrachus has a ME of both the tetrapodan and the teleostean type. The distributional pattern of MAO in the hypothalamus and hypophysis suggests that monoaminergic fibres are probably involved in the secretory mechanism of the releasing factors and pituitary hormones.