An age-dependent effect on the ability to produce cyclic AMP in response to parathyroid hormone (PTH) stimulation was examined using culture skin fibroblasts from both young and elderly female subjects. The mean values of both absolute and comparative production of intracellular cyclic AMP due to various concentrations of PTH administration did not differ between the two age groups, although the mean serum PTH level was significantly higher in the elderly subjects. These results indicate that the decreased response of the target organ to PTH in aged subjects can be explained mainly by the decreased number of intact cells rather than a decreased response of individual cells to PTH.