The utility of measurement of the urinary excretion of the 3-hydroxypyridinium cross-links, pyridinoline (Pyr) and deoxypyridinoline (Dpyr) as indices of bone resorption in rats was investigated. Total Pyr and Dpyr excretion were measured in young rats treated by s.c. infusion with rat parathyroid hormone (1-34) (PTH) at 22-30 micrograms/kg/day or with diluent (controls) for 14 days. During infusion, average urinary excretion of both cross-links was significantly higher in PTH rats (Pyr: 11.77 +/- 0.44 nmol/day), Dpyr: 15.81 +/- 0.95 nmol/day) than in controls (Pyr: 10.17 +/- 0.35 nmol/day, Dpyr: 12.03 +/- 0.67 nmol/day). These results were consistent with the magnitude of the expected increase in bone resorption rate with this dose of PTH. The method appears to provide a sensitive measure of bone resorption for in vivo bone studies in rats.