We used pertussis toxin to study the mechanism(s) by which divalent cations lower cellular cAMP content in bovine parathyroid cells. In cultured parathyroid cells, high extracellular Ca²⁺ (5 mM) or Mg²⁺ (5–10 mMJ lowers dopaminestimulated cAMP content by 70-90%. Pertussis toxin (0.5 ng/ ml) totally blocks the inhibitory effects of Ca²⁺ and Mg²⁺ on cAMP content. Ba²⁺ and Sr²⁺ (5 mM) also lower cAMP content by 80–90%, and this effect is, likewise, blocked by pertussis toxin. Pretreatment with pertussis toxin had no effect on the release of cAMP into the extracellular fluid. The toxin also did not modify phosphodiesterase activity in sonicates of parathyroid cells (42.68 ± 3.26 vs. 47.00 ± 2.82 pmol cAMP hydrolyzed/ 106 cells-20 min in control and toxin-treated cells, respectively). Moreover, addition of the phosphodiesterase inhibitor isobutylmethylxanthine did not modify the inhibition of dopaminestimulated cAMP accumulation by 5 mM Ca²⁺ in control cells (85% vs. 86% inhibition, respectively, with and without isobutylmethylxanthine). Pertussis toxin-catalyzed ADP ribosylation in homogenates of control cells demonstrated the presence of two substrates with mol wt of 40K and 41K. Preexposure of cells to pertussis toxin overnight resulted in the complete loss of both substrates on subsequent ADP ribosylation with [32P]NAD. Pertussis toxin pretreatment did not enhance adenylate cyclase activity indirectly via reducing the extracellular Ca²⁺-induced rise in cytosolic Ca²⁺, since the cytosolic Ca²⁺ level at 5 mM Ca²⁺ was about 60% higher in pertussis toxin-treated than in control cells (531 ± 85 vs. 326 ± 35 nM; P < 0.05). In addition, ionomycin had no significant effect on cellular cAMP levels in control cells despite increasing the cytosolic Ca²⁺ concentration to levels as high as 1700 nM at 10^-5 M. Thus, changes in cytosolic Ca²⁺, phosphodiesterase activity, or efflux of cAMP from the cell cannot explain the inhibition of cAMP accumulation by divalent cations or the reversal of this effect by pertussis toxin. Instead, the present data suggest that extracellular divalent cations modulate the formation of cellular cAMP in parathyroid cells by a process involving a pertussis toxin-sensitive guanine nucleotide regulatory protein, presumably inhibition of adenylate cyclase by G; via a receptor-like mechanism (Endocrinology124: 233–239, 1989)