The antiproliferative properties of melatonin have been previously demonstrated for several normal and tumoral tissues. In a recent report we have shown that melatonin is able to inhibit programmed cell death in thymus both, in vivo and in vitro. Given that other authors have related programmed cell death and cell proliferation and that no previous reports on melatonin and cell division exist on thymus, we decide to study the possible antiproliferative effect of melatonin in this organ measured as the levels of mRNA for the histone H4. We found that melatonin inhibits cell division on thymus when administered chronically both, at high (500 microg/body weight) and low (50 microg/body weight) dose. We also found a circadian rhythm of the mRNA for histone H4, opposed to the one previously described for melatonin, supporting the negative regulation by this hormone of cell division on thymus. A single dose of melatonin (50 microg/body weight) was not able to decrease the levels of mRNA for H4 in the time-points studied but after two hours of its administration. Finally, we report the inhibitory effect of melatonin in the cell proliferation of Harderian gland, brain, lung and kidney.