Radiotracer techniques were used to measure influx and efflux rates of Ca2+ in freshwater-adapted Oreochromis mossambicus. The influx rate of Ca2+ is related to body weight (W) as Fin = 50W0.805 nmol Ca2+/h. For a 20-g fish the calculated influx rate was 558 nmol Ca2+/h, and this was attributed largely to extraintestinal uptake since the drinking rate was estimated to be only 28 microliter water/h, which corresponds to an intake of 22.4 nmol Ca2+/h. The Ca2+ efflux rate was calculated using the initial rate of appearance of radiotracer in the ambient water and the specific activity of plasma Ca2+. Tracer efflux rates were constant over 6-8 h, which indicated that there was no substantial loss of tracer in either the urine or the feces because this would have resulted in random bursts of tracer loss. Efflux rates then primarily represent integumentary and presumably branchial efflux rates. The efflux rate of Ca2+ is related to body weight as Fout = 30W0.563 nmol Ca2+/h, which means an efflux rate of 162 nmol Ca2+/h for a 20-g fish. The net whole-body Ca2+ influx, calculated as Fnet = Fin - Fout, was 396 nmol/h for a 20-g fish, which proves that the ambient water is an important source of Ca2+.