Abstract: In this study, the protective effects of S-allyl cysteine on the temporal patterns of tumor marker enzymes in N-Nitrosodiethylamine (NDEA)-induced hepatocarcinogenesis were studied. Experimental animals (160-180 g) were divided into control, NDEA (single i.p. injection of 200 mg kg-1 body weight followed by weekly subcutaneous injections of 3 mL kg-1 body weight CCl4) treated, NDEA+SAC (200 mg kg-1 body weight) treated and SAC treated groups. The characteristics of circadian rhythms (acrophase, amplitude and mesor) of tumor marker enzymes such as alkaline phosphatase (ALP), aspartate and alanine transaminases (ALT and AST) and γ-Glutamyl transpeptidase (GGT) were analysed. Variations in acrophase, mesor, amplitude and r and p-values were noted. The detectable circadian rhythms of tumor marker enzymes and their alterations during NDEA/SAC treatments, in the present study, deserve further investigation for the diagnosis, prognosis and for the therapeutic efficacy of cancer. As to conclude, this study indicates the necessity of more research to reveal the temporal interplay between the central biological clock (suprachaismatic nucleus), peripheral tissue (liver) based oscillators and cancer processes.