The acute effect of ethanol on amino acid absorption across the in vivo rat intestine was studied using single-pass continuous perfusion and recirculation techniques. The single-pass steady-state perfusion was used to examine the effect on the entire small intestine and recirculation perfusion to examine the effect on short intestinal segments and to limit ethanol absorption. Unlike the in vitro findings of other investigators, ethanol does not cause inhibition of net amino acid absorption in vivo unless the alcohol is perfused in 2 M or higher concentrations. The inhibition that is observed at these concentrations is very likely due to severe injury and shedding of intestinal cells as evidenced by an increased recovery of DNA in the perfusates. The findings suggest that acute ethanol administration, in concentrations that are comparable to those found in the upper intestines of humans after the ingestion of moderate doses of alcohol, does not have a prominent effect on amino acid absorption across the in situ rat intestine. Under these conditions, the ethanol inhibition of active absorption is masked by enhanced diffusion of the amino acids across the intestine.