Table 1. Controversies in the early studies of the effects of melatonin on cancer in rodents (modified from ref. [5]).

References	Experimental system	Results
Tumor inhibition in intact animals
[1]	Transplantable mammary tumor in mice	Inhibition of tumor development
[6]	Solid leukemia in mice (i.m.)	Statistically significant inhibition of tumor size
[7]	Methylcholanthrene-induced sarcoma in mice	Slight delay in occurrence of tumors
[8]	Transplantable mammary tumor in rats	Statistically significant decrease in tumor size
[9]	DMBA-induced mammary tumors in rats	Decreased tumor incidence and tumor growth (melatonin given at 5 p.m.)
Tumor inhibition in pinealectomized animals
[9]	DMBA-induced mammary tumors in rats	Decreased tumor incidence
[10]	Transplantable melanoma in hamsters	Inhibition of tumor development
[7]	Yoshida sarcoma in Wistar rats	Prolonged survival time
No effect in intact animals
[7]	Yoshida sarcoma in Wistar rats	No effect
[11]	Yoshida sarcoma in rats	No effect
[12]	Walker carcinosarcoma in rats	No effect
[9]	DMBA-induced mammary tumors in rats	No effect under constant light
No effect in pinealectomized animals
[9]	DMBA-induced mammary tumors in rats	No effect under constant light
Tumor stimulation in intact animals
[13]	DMBA-induced mammary tumors in rats	Increased tumor incidence and higher degree of malignancy
[7]	Lewis lung carcinoma in mice	Slight reduction of survival time