Maternofetal clearance of 45Ca and 51Cr-EDTA (diffusional marker) were simultaneously measured across in situ perfused placentas of intrauterine growth-retarded (IUGR) and control rat fetuses on d 20 of gestation. IUGR was induced by uterine artery and vein ligation on d 17 of gestation. Control fetuses and their placentas were taken from sham-operated dams. We hypothesized that calcium transfer would be impaired across placentas of IUGR fetuses. The mean body wt of IUGR fetuses was 42% lower, and the mean nose-anus length was 16% lower than those of control fetuses. The mean total calcium content of IUGR fetuses was significantly lower than that of control fetuses, but not when it was normalized to body wt. The mean maternal whole blood ionized calcium concentration was not significantly different in the two groups. The materno-fetal clearance of 45Ca across IUGR placentas was significantly lower than that across control placentas (IUGR = 35.2 +/- 1.9 micro L/min/g placenta, mean +/- SEM; control = 93.1 +/- 12 microliters/min/g placenta, p less than 0.002). In contrast, the maternofetal clearance of 51Cr-EDTA, the reference diffusional marker, was not significantly different across IUGR and control placentas. We conclude that maternofetal transfer of calcium is reduced across placentas of IUGR rat fetuses.