The effect of anticonvulsant drugs on bone growth and calcium metabolism was studied in Wistar strain rats. Animals were treated for 40-48 days with diphenylhydantoin (DPH) or sodium valproate (SV), or were thyroparathyroidectomized (TPTX) and maintained with thyroxine supplements. Bone growth, measured radiographically as increase in bone length, was reduced by up to 12% in the drug-treated and TPTX groups. Plasma iPTH concentrations were raised twofold- to threefold by DPH and SV. Total plasma calcium was not significantly altered in the DPH-treated rats but was elevated by SV treatment. Similarly, elevated iPTH and normal calcium values were also found in carbamazepine-treated and diazepam-treated rats in a separate experiment. Plasma alkaline phosphatase was reduced by high doses of the drugs. These results imply that anticonvulsant drugs induce end-organ resistance to PTH (a feature of pseudohypoparathyroidism), which may be responsible for some of the skeletal and dental abnormalities found in patients treated with anticonvulsants.