Unidirectional flux of inorganic phosphate, Pi, from the umbilical vascular bed perfused in situ into the cell compartment of the guinea-pig placenta, Jfc , was estimated using the indicator dilution method, with 32P-phosphate and 14C-sucrose as the tracers. The flux did not depend on fetal weight; its mean value was 0.25 +/- 0.02 mumol/min (s.e., n = 19). Jfc decreased when the placenta was perfused with CN (10(-3)M) or with Na-free fluid. Elevation of the Pi concentration in the perfusate caused an increase in Jfc which, however, was not proportional. These properties of Jfc were taken to be compatible with a Na-dependent active transport mechanism. The flux in the opposite direction, Jcf , correlated with the fetal weight. It decreased when the umbilical concentration of Pi was elevated. The efflux of 32P from placentae preloaded with the tracer changed inversely with the umbilical concentration of Pi. It is concluded that there is a bidirectional transcellular transfer of Pi between the two sides of the placenta, the flux in the maternal-fetal direction being the prevailing one. The unidirectional transfer to the fetus seems to be regulated directly by the umbilical concentration of Pi.