Prostaglandins (PG’s) stimulated resorption of fetal rat bone in 48–96 hr tissue culture. The effects of several PG’s were compared to the stimulation of bone resorption caused by parathyroid hormone (PTH). PGE₁ and PGE₂ caused increased release of previously incorporated radioactive calcium into the medium, losses of stable and labeled Ca from the bone and morphologic changes of osteoclastic resorption at 10−⁵-10−⁸M. The effects of PGE₁ and PGE₂ were similar to those of PTH (10−⁷- 10−⁸M) but in some experiments the effects of maximal doses of PTH were larger at 48 hr. Like PTH, the effects of PGE₁ were inhibited by thyrocalcitonin and cortisol. PGA₁ and PGF_lα also stimulated bone resorption but only at higher doses (10−⁵M). In vivo studies demonstrated that injections of PGE₁ in parathyroidectomized rats did not increase serum calcium concentration, while parathyroid extract (4–40 U/rat) was effective. (Endocrinology86: 1436, 1970)