We examined the effects of high concentrations of glucose on PTH secretion from cultured bovine parathyroid cells. Increasing medium concentration of glucose caused suppression of PTH secretion. A significant suppression of PTH secretion was found within 48 hours of incubation with as little as 15 mM glucose. The addition of choline chloride to the medium did not suppress PTH secretion, although the osmolality was the same as that of the medium containing 50 mM glucose. When cells previously exposed to 50 mM glucose were reincubated in the medium containing 5 mM glucose for another 48 hours, a complete recovery of PTH secretion was observed. In the cells exposed to 50 mM glucose, the magnitude of the response of PTH secretion to 10-6 M isoproterenol was blunted. Acid-urea gel electrophoresis revealed that the pattern of intact PTH and fragments secreted from cells exposed to high concentration of glucose was similar to that from control cells. Removal of insulin from the medium resulted in a suppression of PTH secretion similar to changes observed with high concentrations of glucose. The suppressive effects of high concentrations of glucose and lack of insulin were additive. However, we cannot exclude from the present studies whether the suppressive effects of the lack of insulin on PTH secretion was secondary to the fact that insulin may be required for the maintenance of parathyroid cells. The present studies demonstrate that glucose directly modulates PTH secretion in primary parathyroid cell culture.