The number of identifiable mast cells and the intraluminal area occupied by porphyrin deposits was studied on semithin sections from female hamster Harderian glands during the estrous cycle and pregnancy. Although the serum levels of estradiol, progesterone, luteinizing hormone and follicle stimulating hormone exhibited significant changes throughout the cycle, no correlation between these changes and the variations in the number of recognizable mast cells was observed. However both during diestrous 1 and proestrous cycles, the number of identifiable mast cells was higher at midnight than at noon (in 14 h light:10 h dark photoperiod with lights on at 07:00 h). A more exhaustive study revealed the presence of 'degranulated mast cells' which were not stained with toluidine blue. Thus, a diurnal cycle in degranulation might occur in the Harderian glands from female hamsters. No significant variations were observed in the area occupied by intraluminal porphyrin deposits during the estrous cycle. However, both the relative number of mast cells and the area occupied by intraluminal porphyrins decreased from day 4 of pregnancy to day 14 showing a strong correlation. The Harderian glands from female Syrian hamsters might provide a useful model for the study of mast cell degranulation during porphyria.