Alcohol dehydrogenase (ADH) activities were measured in rectal biopsies from 55 patients (28 males, 27 females aged 22-81 years), in colonic biopsies from 19 patients (10 males, 9 females aged 21-81 years) and in three surgical specimens. All patients had normal mucosa as determined by light microscopy. The activity of rectal ADH was comparable to gastric ADH activity and did not exhibit any significant gender effect (5.5 +/- 1.1 vs. 6.7 +/- 1.0 nmol/mg protein x min; nonsignificant). No significant correlation was found between age and rectal ADH activity. Compared to ADH activities in other colonic segments, rectal ADH activity was found to be significantly increased (ascending colon: 3.9 +/- 0.7 nmol/mg protein x min; p < 0.05; transversal colon: 3.4 +/- 1.1 nmol/mg protein x min; p < 0.05; descending colon 2.3 +/- 0.4 nmol/mg protein x min; p < 0.001; rectum 6.1 +/- 0.8 nmol/mg protein x min). This higher activity of ADH in the rectum could result in increased acetaldehyde levels after alcohol administration and could therefore play a role, at least in part, in the ethanol-associated rectal cocarcinogenesis.