Murdannia loriformis is a medicinal plant used in treatment of bronchitis and relieve ailment of neoplasm. Previous study has shown that 80% ethanolic extract of M. loriformis had antimutagenicity against various kinds of mutagens on Salmonella typhimurium, TA98 and TA100. In the present study, effects of 80% ethanolic extract of M. loriformis extract on DNA adduct and aberrant crypt foci (ACF) formations in rat colons were examined. One week after M. loriformis extract administration at the doses of 0.1 and 1.0 g/kg bw. by gavage, male F344 rats received two subcutaneous injection of azoxymethane (AOM) 15 mg/kg bw. one week apart. The rats were sacrificed 12 hr. after the second AOM injection. Colons were removed for DNA extraction and analyzed for O6-methylguanine and N7-methylguanine adducts formation by High Performance Liquid Chromatography (HPLC). To determine an inhibitory effect of the plant extract on initiation stage, AOM-treated rats were continuously received the extracts and were killed 3 weeks after the second AOM injection. To study effect of the plant on the promotion stage, rats were fed with the plant extract 2 weeks after the second AOM injection and continued for 12 weeks and were sacrificed. It was found that M. loriformis extract reduced AOM-induced DNA adducts formation but not significant. In addition, M. loriformis extract significantly inhibited ACF formation both at the initiation and promotion stages of colon carcinogenesis at 56% and 17%, respectively. Interestingly, treatment with M. loriformis extract could inhibit the formation of larger ACF (with four or more crypts per focus) at the promotion stage about 27%. The effects of M. loriformis extract on induction of xenobiotic-metabolizing enzymes in male Wistar rats and antioxidant activity of the extract were also studied. The rats received the extract at dose 0.1 and 1.0 g/kg bw. for 30 days. Ten rats per group were sacrificed after receiving the extract for 10 days and 30 days. The results showed that after administration of the extract for 10 days aminopyrine-N-demethylase (APD) and UDP-glucuronyl transferase (UDP-GT) activities in rat livers were significantly decreased. On the contrary, after 30 day-administration resulted in a significantly induced APD, glutathione S-transferase and UDP-GT activities. The antioxidant activity was determined by using t-butylhydroperoxide-induced malondialdehyde formation. The extract showed mild antioxidant activity. It was suggested that M. loriformis extract affect at the initiation phase of colon carcinogenesis, but not growth or differentiation in promotion stage. The possible mechanism of chemoprotection of M. loriformis extract at initiation stage may be through selective alteration of enzymes in xenobiotic-metabolizing systems. Its antioxidant activity may be involved at promotion stage.