The distribution of carbaryl labeled with 14C in the ring, methyl or carbonyl groups was determined in pregnant mice and rats. Three identical concurrent experiments were performed using each of the three radiolabeled compounds in each of the species so that the different moieties of carbaryl could be traced through maternal tissues and across the placenta to the fetus for both assay of activity and identification of carbaryl. Following a single dose in the maternal tissues, the concentration of carbaryl 14C-equivalents peaked at 1-2 hours and declined steadily to 24 hours for all three labels in the mice. In rats, the naphthyl label concentration declined steadily for 24 hours but not that of the methyl or carbonyl label. The naphthyl label was the best estimator of carbaryl concentration in both mice and rats. In the fetuses and placentas of mice and rats both carbaryl and metabolites were detected. The fetuses and placentas retained the methyl and carbonyl labeled groups and use of these labels could result in erroneously high estimates of carbaryl.