A survey on the distribution of mast cells in healthy rats was carried out. Mast cells were present in abundance in mesenteric lymph nodes and tongue and to a lesser extent in sciatic nerve. Fewer numbers of mast cells were seen in other lymph nodes, heart, salivary gland, thymus, pancreas, cervix, vagina, uterus, epididymis, liver, skeletal muscle, ovaries, prostate, seminal vesicles, harderian glands, parathyroid, thyroid, mammary gland, skin, adipose tissue, preputial gland, tail and bone marrow. Scattered individual mast cells were occasionally seen in submucosa and serosa of stomach, oesophagus, small and large intestine and urinary bladder. Mast cells were very rare in lung, spleen and adrenals. Mast cells were not seen in kidney, pituitary, testes, bone spinal cord, brain, optic nerves and eyes. In lymph nodes, mast cells appeared mainly in medullary sinusoids with varying extent. There was no clear evidence of a difference between numbers of mast cells in young and older animals. There was evidence of increased numbers of mast cells in inflammatory lesions, of skin and heart. Mast cells were seen associated with mammary tumours, fibromas and fibrosarcomas. There were more mast cells in rats than in mice. Mast cells appear to be more abundant in Wistar rats than in CD rats. Mast cells were hardly seen in healthy dogs and primates except a few in tongue and sciatic nerve. Presence of mast cells is incidental and not indicative of any abnormalities.