The influence of age and sex on the intestinal in vitro absorption of the trace elements zinc and manganese was studied with SPF-Sprague-Dawley-rats. For this purpose the absorption was measured with animals of both sexes starting at once after the weaning period (17 to 20 days) until an age of maximal 80 days. The measurements were made continuously in periods of a few days. The results of a total of five experimental series show that the transfer to the serosal solution and the uptake of the intestinal wall of both elements decrease permanently with increasing age. The most severe decrease occurs, however, at an age of the rats between 20 and 40 days, that is shortly after the weaning period. The previous change of the diet, however, cannot be the main explanation for the significant alteration of the absorption. In order to test this possibility the rats were only fed with the milk of their mothers up to an age of 36 days, while the control groups received already a pelleted casein diet starting with an age of 18 days. The results of these experiments were that the milk-fed rats absorbed only somewhat more zinc or manganese than did the corresponding control groups. There were no significant differences in the amounts of Zn absorption between male and female rats.