High alcohol and low folate intake are independent risk factors for colorectal cancer. Acetaldehyde has been postulated to be a factor responsible for ethanol-associated carcinogenesis. High levels of acetaldehyde accumulate in the large intestine via the microbial oxidation of alcohol. Acetaldehyde degrades folate in vitro. Thus, it is possible that high intracolonic acetaldehyde levels break down folate in the colon. Our aim was to test the effect of high alcohol and acetaldehyde concentrations in the gut on systemic and local intestinal folate levels in rats. Twenty rats received 3 g/kg of ethanol twice a day for 2 weeks with or without concomitant ciprofloxacin administration. Twenty control rats received saline with or without ciprofloxacin. All rats were fed a diet with normal folate content. Alcohol treatment led to very high intracolonic acetaldehyde levels (387 +/- 185 microM), which were markedly decreased by concomitant ciprofloxacin treatment (21 +/- 4 microM). Erythrocyte, serum and small intestinal folate levels were unaffected by alcohol treatment. Alcohol administration decreased significantly colonic mucosal folate levels by 48%, and this effect was prevented by ciprofloxacin. We conclude that alcohol administration for 2 weeks leads to local folate deficiency of colonic mucosa in rats, most probably via the degradation of folate by the high levels of acetaldehyde microbially produced from ethanol. Our findings offer a unique explanation for the increased risk of colonic cancer associated with alcohol intake and folate deficiency.