The aim of the present study was to evaluate melatonin effects on the callus induction and phenolic compound production of Ocimum basilicum L. (sweet basil). Calluses, derived from leaf explants, were grown on Murashige and Skoog (MS) medium supplemented with 0, 100, or 200 mu M melatonin, and subsequently extracted for determination of their phenolic contents. Melatonin decreased the callus induction in both concentrations. Based on the phytochemical analysis, the highest total phenolic acid contents (784.6 mu g g(-1) and 335.2 mu g g(-1), respectively) were recorded in calluses grown in 100 and 200 mu M melatonin-supplemented medium, compared with the calluses induced with MS alone (192.0 mu g g(-1)). Among the five phenolic acids confirmed in the callus samples, rosmarinic acid was the major constituent. The amount of rosmarinic acid increased significantly in callus grown on 100 mu M melatonin medium by nearly 5-fold (754.2 mu g g(-1)), compared with the control group callus. Major volatiles in basil calluses were represented by 3-methylbutanal, benzaldehyde, 1,8-cineole, 2-nonenal, eugenol, and methyl eugenol, and these were in the ranges of 4 to 14%, 24 to 50%, 2 to 3%, 0 to 0.55%, and 2 to 17% (in relative percentages), respectively. The qualitative and quantitative analyses of these substances found in calluses formed on melatonin-supplemented or melatonin-free medium were evaluated separately.