The aim of this work was to investigate if the possible chemopreventive effect of aspirin (ASA) on rat colon carcinogenesis could be detected with a medium-term assay. The end-point chosen was the inhibition of incidence and growth of putative preneoplastic lesions, the aberrant crypt foci (ACF) induced in the rat colon by two administrations of 1,2-dimethylhydrazine (DMH, 25 mg/kg p.o.). At both 4 and 8 weeks after the starting of the carcinogenic treatment the incidence of total ACF was reduced of 60% in rats receiving ASA (10 mg/kg/day p.o.) for 12 consecutive days during the initiation treatment with DMH. Also the number of the larger foci (with 3 or more crypts) was significantly lower in ASA-treated rats at both time-points (about 70% reduction). Moreover, concomitant ASA treatment determined a significant decrease of the mean number of crypts per focus at week 8. These results indicate that the chemopreventive effect of ASA on chemically-induced rat colon carcinogenesis observed in long-term studies may be detected in this relatively short assay.