Male Wistar rats (188 g) were fed on a semi-synthetic (SS) diet (38 mg iron/kg) ad lib. for 7 d and then meal-fed for 1 d. After a 21 h fast each rat was given one meal (10 g) of high-Fe SS (500 mg Fe/kg; high-Fe group) or control (38 mg Fe/kg; control group) diet. After 16 h 2 ml of an 59Fe-labelled ferrous sulphate solution (18 kBq 59Fe; 120 micrograms Fe) was administrated by gavage and equal numbers of rats from each group were killed 6 or 24 h after dosing. Mucosal uptake of 59Fe from the gut lumen and transfer of 59Fe from mucosa into the carcass were measured. Total Fe content of the duodenum was also determined. Mucosal 59Fe uptake and transfer were markedly lower in the high-Fe group compared with the control group. The Fe content of the duodenum, the major region of Fe absorption, was significantly greater in the high-Fe group than in the controls. A larger amount of Fe may thus have been released into the lumen of the high-Fe rats, via mucosal cell turnover, resulting in a greater lumen dilution of the 59Fe dose in this group compared with the controls. Calculations are presented which demonstrate that such an effect could not possibly account for the observed difference in mucosal 59Fe uptake between groups.(ABSTRACT TRUNCATED AT 250 WORDS)