Ocimum sanctum L., a plant having multi-medicinal properties, has been investigated for its chemopreventive activity against 7,12-dimethylbenz (a) anthracene (DMBA)-induced hamster buccal pouch carcinogenesis. O. sanctum, in the form of fresh leaf paste, aqueous extract and ethanolic extract were topically applied and the extracts were orally administered to buccal pouch mucosa of animals exposed to 0.5% of DMBA. Incidence of papillomas and squamous cell carcinomas were significantly reduced, and increased the survival rate in the topically applied leaf paste and orally administered extracts to animals. Among them, the orally administered aqueous extract showed profound effect than the other two forms. Histopathological observations made on the mucosa confirmed these findings. Further fluorescent spectral studies at 405 nm excitation on the mucosa of control, DMBA and extracts orally administered experimental animals showed a prominent maxima at 430 nm for control, 628 nm for DMBA induced carcinomas while aqueous and ethanolic extracts administered animals showed at 486 nm and 488 nm, respectively. The fluorescent intensity at 630 nm (FI630 nm) was significantly reduced and the ratio of fluorescent intensities at 520 nm and 630 nm (FI520 nm/630 nm) were significantly increased in orally administered extracts compared to DMBA treated animals. These observations suggest that the orally administered extract of O. sanctum may have the ability to prevent the early events of carcinogenesis.