Experiments were performed on 36 plasma-expanded Munich-Wistar rats to examine the effects of acute hypercalcemia on the determinants of glomerular ultrafiltration. Elevation of total plasma calcium concentration to an average value of 13.2 +/- 0.5 mg/dl, by acute infusion of calcium chloride into nonthyroparathyroidectomized (non-TPTX) rats, resulted in significant declines in single nephron and whole kidney glomerular filtration rate. These declines were due primarily to a fall in the glomerular capillary ultrafiltration coefficient (Kf), to a mean value approximately 60% below that determined in the pre-infusion period. These changes were not seen in a separate group of sham-treated non-TPTX rats. It is of interest that these effects of acute hypercalcemia were largely abolished in rats that underwent acute TPTX before hypercalcemia. Infusion of a submaximally phosphaturic dose of parathyroid hormone, together with calcium chloride, into a second group of acute TPTX rats, however, had the effect of reproducing the striking declines in filtration rate and Kf noted in non-TPTX rats given calcium chloride alone. These findings suggest that the decline in filtration rate associated with hypercalcemia is due largely to the reduction in Kf, the latter dependent upon the presence of parathyroid hormone.