Weanling rats were given a low Ca (0.003%)/high P (0.64%) diet with and without vitamin D for periods up to 5 weeks. This was associated with hypocalcaemia, rachitic bone changes and increased bone resorption. These changes preceded the accumulation of large numbers of mast cells in long bone metaphyses. Mast cells did not increase in the epiphyses of long bones or in caudal vertebrae, tooth pulp, skin and other organs. A light and electron microscopic study showed that most mast cells had raised secretory activity, as evidenced by variability in the structure of granules and loss of granule contents, particularly in animals with the lowest serum calcium levels. It was not possible to relate the position of a mast cell to an area of active bone formation or resorption. The increase in mast cells might be related more to the maintenance of connective tissue integrity in areas of rapid bone remodelling.