The distribution of Mast Cells (MCs) was studied in the lymphoid organs of 7, 14, 21 and 30 days old quails (Coturnix coturnix Japonica) kept in different photoperiods using light microscopy histochemical techniques. The distribution of MCs was determined in different age groups housed in continuous light (23L:1D) or in a light-dark regimen (18L: 61)). Tissue samples were obtained under deep anesthesia from birds in the four age groups, fixed in Mota's fixative (basic lead acetate) for 24 h and embedded in paraffin. Six micrometre-thick sections were stained with 0.5% toluidine blue and the number of MCs counted under the microscope. The numbers of MCs were significantly different between both age and light treatment groups. The number of mast cells was significantly lower (p<0.05) in birds exposed to continuous light (23L:1D) than in birds exposed to a shorter light period (18L:6D). Photoperiod was concluded to influence mast cell numbers in the lymphoid organs of the Japanese quail and thus the immune response of the birds.