The daily administration of 25 micrograms of melatonin for 10 weeks resulted in an increase in the percentage of Type II cells in the Harderian glands of male Syrian hamsters. Harderian glands of melatonin injected animals consisted of 65-70% Type II cells while control animals which were injected with saline had 40% Type II secretory cells. The daily administration of 3 mg of the glutamate receptor agonist N-methyl-D-aspartate (NMDA) prevented the effects of melatonin on cell differentiation but was without effect when administered to saline treated hamsters alone. Both the relative number of mitoses and the number of total cells, estimated by counting the nuclei, was not affected. Thus, a conversion from Type I to Type II cells seems possible. The effects of melatonin and NMDA administration were independent of the serum levels of testosterone, luteinizing hormone and thyroxine, hormones which have been implicated in Type II cell differentiation. However, prolactin levels, which were affected by melatonin and NMDA administration, might be involved in the differentiation of Harderian gland secretory cells.