['The role of parathyroid hormone (PTH) in the rapid minute-to-minute regulation of plasma Ca2+ (p-Ca2+) was studied in vivo in rats.', 'The rapid calcaemic response to exogenous rat PTH1-34 (16 microg) was examined in normal rats, and the long-term calcaemic response was examined in parathyroidectomized (PTX) rats receiving PTH1-34 for 24 h at 0.2, 0. 4 and 0.8 microg h-1. Acute hypocalcaemia was induced by EGTA for 30 min, and then the rapid recovery of p-Ca2+ was studied for 130 min in normal rats, 24 h after PTX and in PTX rats infused with exogenous rat PTH1-34. The dynamics of the rapid recovery of p-Ca2+ was studied at two additional doses of EGTA.', 'No rapid calcaemic response was observed in the first 60 min after administration of PTH and no hypocalcaemia was seen for 2 h after acute PTX. This slow effect of PTH suggests that PTH might not be responsible for maintaining the stable p-Ca2+ on a rapid minute-to-minute basis. EGTA induced acute hypocalcaemia in both normal and PTX rats (P < 0.01). In both groups a rapid and similar increase in p-Ca2+ took place 10 min after discontinuing EGTA (P < 0. 05). Within 60 min, p-Ca2+ increased further, independently of the presence of PTH. Infusion of PTH to PTX rats did not affect the rapid recovery of p-Ca2+ (P < 0.05) from EGTA induced hypocalcaemia.'] Is parathyroid hormone a key hormone in the rapid minute-to-minute regulation of plasma Ca2+ homeostasis in rats?