Parathyroid glands from inbred rats were transplanted to rats of the same strain. The transplantation resulted in hyperparathyroidism and persistent hypercalcaemia. These hypercalcaemic animals were compared with the hypocalcaemic donors and with untreated controls. No significant differences in serum gastrin values were found between the different groups. In all three groups, one series of animals was killed 6 weeks after the transplantation (6w series) and another after 14 weeks (14w series). Quantitative studies of the antral gastrin cells showed an increase in the number of these cells per unit volume in the 6w series in the hyperparathyroid recipient animals. The number of gastrin cells per unit segment was also higher in the recipient animals than in the parathyroidectomized and untreated groups in both the 6w and 14w series. The amount of gastrin extracted from antral mucosa did not differ between the different groups. The findings show that an experimentally induced hyperparathyroidism in the rat gives a transient increase in the number of antral gastrin cells but no changes in the antral gastrin or serum gastrin levels.