We employed the calcium (Ca++)-sensitive, intracellular dye QUIN-2 to examine the role of cytosolic Ca++ in the stimulation of PTH release by high extracellular potassium (K+) concentrations. Addition of 55 mM KC1 to cells incubated with 115 mM NaC1 and 5 mM KC1 lowered cytosolic Ca++ at either low (0.5 mM) extracellular Ca++ (from 194 +/- 14 to 159 +/- 9 nM, p less than .01, N = 6) or high (1.5 mM) extracellular calcium (from 465 +/- 38 to 293 +/- 20 nM, p less than .01, N = 10). This reduction in cytosolic Ca++ was due to high K+ per se and not to changes in tonicity since addition of 55 mM NaC1 was without effect while a similar decrease in cytosolic Ca++ occurred when cells were resuspended in 60 mM NaC1 and 60 mM KC1. PTH release was significantly (p less than .01) greater at 0.5 and 1.5 mM Ca++ in QUIN-2-loaded cells incubated with 60 mM NaC1 and 60 mM KC1 than in those exposed to 115 mM NaC1 and 5 mM KC1. In contrast to most secretory cells, therefore, stimulation of PTH release by high K+ is associated with a decrease rather than an increase in cytosolic Ca++.