The heart of the Oreochromis niloticus has been studied ultrastructurally and immunohistochemically in order to describe the distribution of ANP producing cells, and the nature of the putative fish cardiac hormone. Utrastructural examination of the bulbus arteriosus, atrial and ventricular mycoardial layer has disclosed the presence of at least four cell types:1)
ا working myocardiocytes; 2) granules-containing contractile cells; 3) Purkinje conduction cells and 4) granules-containing non-contractile cells. The granules-containing contractile cells were observed as an isolated endocrine units randomly scattered throughout the different myocardial regions. In comparison to other mammalian species, the secretory activity of both the granules-containing contractile cells and the conduction cells was ill developed. Negative anti- human ANP immunoreactivity was demonstrated for all cell types. In conclusion, the heart of the Oreochromis niloticus seemed to have a less pronounced ANP secretory activity. Moreover, the nature of the fish cardiac hormone was apparently different form that of the human ANP