The roles of a trimeric GTP-binding regulatory protein, protein kinase A and mitochondria in the regulation of store-activated (thapsigargin-stimulated) Ca2+ inflow in freshly-isolated rat hepatocytes were investigated. Rates of Ca2+ inflow were estimated by measuring the increase in the fluorescence of intracellular fura-2 following the addition of extracellular Ca2+ (Ca2t0) to cells incubated in the absence of added Ca2t0. Guanosine 5′-[γ-thio]-triphosphate (GTP[S]) and AlF4- inhibited the thapsigargin-stimulated Ca2+0 induced increase in cytoplasmic free Ca2+ concentration ([Ca2+]c) and this inhibition was prevented by the Rp diastereoisomer of adenosine 3′,5′-(cyclic)-phosphoro[thioate]. cAMP, forskolin and glucagon (half-maximal effect at 10 nM) mimicked inhibition of the thapsigargin-stimulated Ca2+0-induced increase in [Ca2+]c by GTP[S], but had little effect on thapsigargin-induced release of Ca2+ from intracellular stores. Azide and carbonyl cyanide p-trifluoromethoxyphenylhydrazone inhibited the thapsigargin-stimulated Ca2+-induced increase in [Ca2+]c in the presence of increased cAMP (induced by glucagon). In contrast, Ruthenium Red markedly enhanced the thapsigargin-stimulatcd Ca2+0-induced increase in [Ca2+]c in both the presence and absence of increased cAMP (induced by forskolin and dibutyryl cAMP). It is concluded that, in hepatocytes, protein kinase A regulates the disposition of Ca2+, which enters the cytoplasmic space through store-activated Ca2+ channels, by directing some of this Ca2+ to the mitochondria. The idea that caution should be exercised in using observed values of Ca2+0-induced increase in [Ca2+]c as estimates of rates of agonist-stimulated Ca2+ inflow is briefly discussed.