We studied the effect of parathyroid hormone (PTH) on the in vitro conversion of 25-hydroxycholecalciferol to 1,25-dihydroxycholecalciferol [1,25-(OH)₂D₃] by kidney slices from vitamin D-deficient chicks. Bovine PTH (bPTH) stimulated 1,25-(OH)₂D₃ production at low concentrations, with maximal stimulation (65%) at a concentration of 25 ng/ml bPTH in the absence of theophylline. Higher concentrations of bPTH resulted in less stimulation. The addition of 5 mM theophylline to the incubation buffer decreased basal 1,25-(OH)₂D₃ production but potentiated the stimulation of 1,25-(OH)₂D₃ production by PTH. Maximal stimulation (170%) was observed with 2 ng/ml bPTH in the presence of theophylline. Maximal stimulation of cAMP production by the kidney slices required 2- to 3-fold larger concentrations of bPTH. However, cAMP by itself stimulated 1,25-(OH)₂D₃ production, with maximal stimulation (70%) at 10^-7-10^-5 M cAMP. We conclude that stimulation by PTH of l,25-(OH)₂D₃ production can be potentiated by theophylline and mimicked by cAMP. However, such stimulation occurs at PTH concentrations lower than that required for optimal stimulation of adenylate cyclase activity. (Endocrinology108: 1002, 1981)