Effects of prolactin on morphology and numbers of chloride cells in the opercular membrane of seawater-adapted tilapia (Oreochromis mossambicus) have been examined. Following five daily injections of ovine prolactin at a dose of 10 μg · g body wt⁻¹, blood samples were taken and opercular membranes were removed and stained with a fluorescent mitochondrial dye (dimethylaminostyrylethylpyridiniumiodine), a fluorescent derivative of ouabain (anthroylouabain), and a histological stain specific for the extensive tubular system of chloride cells (zinc-osmium-iodine). Mean plasma osmolarity and sodium increased 23–24% following prolactin injection. An increase in the relative frequency of chloride cells between 20 and 180 μm² in cross-sectional area and a decrease in the relative frequency of chloride cells greater than 180 μm² were observed following prolactin injections. Average cell size decreased 46–70% and cell height decreased 26–38% following prolactin injections. There was no significant change in cell density. Anthroylouabain staining was observed in both prolactin- and saline-injected fish, and no significant effect on Na⁺,K⁺-adenosinetri-phosphatase activity was seen in either opercular membrane or gill tissue. The results demonstrate an effect of prolactin on chloride cell size and provide a morphological correlate for decreased secretory activity of chloride cells following prolactin injections.