The aim of the present study was the demonstration of mechanisms of regulation of activity of chromaffin cells in the adrenal gland of Rana ridibunda (Anura-Amphibia). Previous studies have shown that endothelin-1 is an important factor for the maintenance of adrenal gland function. On the basis of these findings, frogs were injected with [Ala(1,3,11,15)] -endothetin-1 (0.025 mg/0.2 ml), which is a selective agonist of the endothelin B receptor, whereas control. animals were injected with Ringer solution (0.2ml). The adrenal glands were removed at 5, 20, and 60min after injection and fixed, embedded in paraffin wax and stained by histotogical. and immunohistochemical means, applied on adjacent 4-mu m-thick sections. Sections were stained with hematoxytin and eosin for overall tissue analysis and, in parallel, serotonin was localized using the streptavidin-biotin complex technique white doparnine beta-hydroxylase and serotonin 2A receptors were shown by the peroxidase-antiperoxidase (PAP)-3,3'-diaminobenzidine tetrachloride (DAB) method. After injection of [Ata(1,3,11,15)]-endothelin-1, chrornaffin cells secreted serotonin and synthesized dopamine beta-hydroxytase. In conclusion, these findings suggest that [Ala(1,3,11,15)]-endothelin-1 stimulates chromaffin cell activity in frog adrenal glands. Moreover, the presence of serotonin 2A receptors in chrornaffin cells indicates that these cells are also targets for serotonin and that there is an autocrine signaling pathway in chromaffin cells. This is the first report providing data on the effects of endothelin-1 on chromaffin cells in frog adrenal glands. (c) 2005 Elsevier GmbH. All rights reserved.