It has been shown that both parathyroid hormone (PTH) and calcitonin (CT) increase the concentration of cyclic 3′,5′-adenosine monophosphate (cyclic AMP) in skeletal tissue. Since these two hormones have opposing actions on bone resorption, it has been postulated that the cell types upon which the hormones act in skeletal tissue may be different. We have previously shown that osteoblasts and osteocytes contain adenylate cyclase responsive to both PTH and CT whereas the enzyme prepared from periosteum and marrow cells did not respond to either. To further examine the postulate, the effect of these hormones on the concentration of cyclic AMP was determined. Bovine PTH (5 U⁄ml = 4.0 μg⁄ml ≅ 4.2 x 10-7M) significantly increased the concentration of cyclic AMP⁄μg DNA in periosteum, osteoblasts, and osteocytes but not in marrow cells. Porcine CT (0.5 U⁄ml = 8.0 μg⁄ml ≅ 3.0 x 10-6M) significantly increased the concentration of cyclic AMP⁄μg DNA in periosteum, osteoblasts, osteocytes, and marrow cells. Adenylate cyclase activity prepared from periosteum and marrow cells was re-examined using EGTA and DMSO to improve enzyme stability and activity. With these additions PTH (5 U⁄70 μl) increased activity of the preparation from periosteum, and CT (0.5 U⁄70 μl) increased activity from marrow cells and periosteum. These studies provide evidence that: 1) periosteum, osteoblasts, and osteocytes respond directly to both PTH and CT and 2) marrow cells respond to CT and not PTH.(Endocrinology96: 1261, 1975)