The second messengers for PTH action on fi-broblasts have not been determined. The hypothesis that Ca2+is a second messenger was examined by spectrofluorometric measurement of cytosolic calcium ([Ca2+]i) of fura-2-loaded hu-man dermal skin fibroblasts. PTH induced: 1) transient eleva-tions of [Ca2+]idue to both Ca2+influx and intracellular release which were independent of CAMP generation, and 2) membrane depolarization. PTH treatment of fibroblasts depolarized by KCl induced oscillations of [Ca2+]i(but spontaneous oscillatory activity was not observed. Transient elevations of [Ca2+]isimilar to that induced by PTH were observed with PTH-related peptide (PTHrp). PTH regulation of [Ca2+]iwas normal in fibroblasts from patients with pseudohypoparathyroidism (PHP) type Ia (deficiency of the stimulatory GTP-binding protein, Ga), but PTH induced transient decreases in [Ca2+]iin fibroblasts from patients with PHP type Ib (defective PTH receptor). Both PHP Ia and Ib fibroblasts exhibited: 1) spontaneous [Ca2+]ioscilla-tions, possibly due to voltage gated Ca2+entry and Ca2+induced Ca2+release; and 2) greater sensitivity than normal fibroblasts for release of Ca2+from intracellular stores when [Ca2+]iwas decreased by lowering external Ca2+. Conclusions: 1) PTH- induced transient elevations in [Ca2+]iin normal fibroblasts result from Ca2+entry and intracellular release which are independent of CAMP generation; 2) [Ca2+]ihomeostasis is altered in PHP fibroblasts, resulting in Ca2+oscillations; 3) PTH regulation of [Ca2+]iis altered in PHP Ib, but not in PHP la fibroblasts.