The stimulatory effect of parathyroid hormone (PTH) on renal la-hydroxylation of 25- hydroxyvitamin D₃ (25-OH-D₃) was studied in thyroparathyroidectomized (TPTX), vitamin D-deficient rats into which bovine PTH, theophylline, cAMP or dibutyryl cAMP (dbcAMP) was constantly infused. The accumulation in plasma of 1α,25-dihydroxy vitamin D₃ [1α,25-(OH)_2⁻D₃], produced from 25- OH-D₃, was enhanced by infusion of either cAMP (0.9 μmol/h) or dbcAMP (1 μmol/h) to a level similar to the maximum obtained by PTH (2.5-7.5 U/h) infusion. A submaximal dose (1 U/h) of PTH caused a similar extent of stimulation, when infused with theophylline. When either 2 μmol/h of cAMP or 7.5 U/h of bovine PTH was infused starting 18 h after TPTX, the accumulation of lα,25-(OH)_2⁻D₃ in plasma was similarly restored within 6 h to the level found in the sham-operated animals. These results strongly support the concept that cAMP plays an important intermediary role in the stimulation of lα,25-(OH)_2⁻D₃ production induced by both exogenous and endogenous PTH.