1. The value of progestogen therapy in the prevention of postmenopausal bone loss was assessed in 30 women, by a preliminary randomized controlled trial of gestronol or mestranol, in comparison with a placebo. 2. When the skeletal response was measured by photon absorptiometry, bone mineral loss was prevented by both the oestrogen and the progestogen. 3. We confirm that mestranol significantly reduced the urinary output of hydroxyproline-containing peptide, but this did not occur during gestronol therapy, suggesting that progestogen has a different action on bone, perhaps stimulating bone formation.