In this study we investigated the effect of volume expansion on plasma and atrial concentrations of atrial natriuretic peptide (ANP) in the presence and absence of the parathyroid gland and under normocalcemic and hypocalcemic conditions. After volume expansion ANP concentration in plasma was significantly (p < 0.001) higher in intact (702 ± 86 pg/ml) than in hypocalcemic parathyroidectomized (PTX) (271 ± 38 pg/ml) rats. Plasma ANP of PTX rats rendered normocalcemic with oral calcium supplementation increased to 402 ± 85 pg/ml after volume expansion. Results from this study suggest that parathyroid hormone (PTH) is required for augmented ANP secretion in response to acute volume loading and alterations of extracellular calcium may modulate volume-induced ANP release in PTX rats. We would discuss that a parathyroid gland-cardiac atria interaction exists and that changes in serum level of PTH may play a role in the regulation of fluid homeostasis via ANP secreti