The influence of ethyl 3-(4'-geranyloxy-3'-methoxyphenyl)-2-propenoate (EGMP) on the initiation and post-initiation stages of colon carcinogenesis was investigated in male F344 rats treated with azoxymethane (AOM). In experimental protocol 1, EGMP was given in the diet at 0.1 or 0.2% for 1 week together with two s.c. 15 mg/kg body weight injections of AOM on days 1 and 7 (initiation period). In protocol 2, the test compound was administered starting at week 3(post initiation stage), and in protocol 3, the test compound was given throughout the experimental period(whole stage). Sacrifice and quantitation of aberrant crypt foci (ACF) was performed at the end of week 5. Dose-dependent decreases in numbers of ACF were noted with both cases of post-initiation and whole period exposure (protocol 2 and 3), large size lesions considered most likely to be precursor lesions also being significantly reduced in the protocol 2(4-9 crypt size total with the 0.2% dose group, 48.9% and 59.6% of control values, respectively). No effects on body or liver weights were evident. The present results thus suggest that EGMP might find application as a chemopreventive agent against colon tumor development.