The distribution of [14C]caprolactam was studied by whole-body autoradiography in male, female and 14.5-day-pregnant mice. This technique does not allow translocation or removal of soluble compounds from the sites of localization. Pregnant mice were frozen 20 min and 1, 3, 9 and 24 hr after oral administration of the compound. The non-pregnant mouse was frozen 3 hr after oral dosing; two male mice were frozen 20 min and 9 hr after intravenous administration. Radioactivity was rapidly absorbed from the stomach and distributed throughout the entire animal, including the foetuses. There was efficient elimination by the kidney and liver. Material secreted by the liver into bile and intestinal contents appeared not to be reabsorbed via the enterohepatic circulation. The kinetics of distribution and elimination appeared to be the same in male, female and pregnant animals. The only sites of retention of radioactivity after 24 hr were the umbilical cords, amnion, yolk sac, maternal lens, maternal Harderian gland and maternal liver. The distribution into and removal from the foetuses was typical of molecules that diffuse freely across the placenta. There was no retention of radioactivity in any foetal tissue. With the possible exception of some residual activity in the nasal epithelium, no localization was seen that would suggest a site of toxic action of caprolactam.