Parathyroid hormone, calcitonin, and prostaglandin E2 activate the adenylate cyclase-cyclic AMP system in fetal-rat calvaria. These agents presumably interact with the tissue at separate receptor sites. When calvaria were preincubated with trypsin, 500 μg/ml for 45 min, the subsequent increase in 3′,5′-AMP in response to parathyroid hormone was markedly diminished, whereas the response to calcitonin and prostaglandin E2 were not altered significantly. The effect was attributable to an action of the enzyme on the tissue and not to hydrolysis of the hormone. Similarily, preincubation of calvaria with trypsin prior to homogenization and preparation of a crude plasma membrane fraction decreased PTH-sensitive adenylate-cyclase activity by 58% but did not alter the degree of stimulation of the enzyme in response to calcitonin, prostaglandin E2, or sodium fluoride. These studies support the hypothesis that the actions of parathyroid hormone and calcitonin on bone are mediated through distinct receptor sites, and the receptors for parathyroid hormone can be altered selectively with trypsin.