We tested the effect of ethanol and its metabolite, acetaldehyde, on bone formation as measured by [ 3H]proline incorporation into collagenase digestible protein (CDP) and noncollagen protein (NCP), and on DNA synthesis as measured by [ 3H]thymidine (TdR) incorporation in fetal rat calvaria. We also determined the effects of ethanol and acetaldehye on prostaglandin E 2 (PGE 2) release from calvaria and on bone resorption as measured by 45Ca release from fetal rat long bones. Bones were cultured in multiwell plastic dishes (open system) or in stoppered Erlenmeyer flasks (closed system) for 24 to 96 h. In the open system, 1% ethanol (v/v; 172 mM) resulted in a 31% decrease in TdR incorporation at 24 h with no effect on CDP and NCP. At 0.1% (17.2 mM), ethanol increased TdR by 22%, CDP by 73% and NCP by 67% at 24 h, but these effects were not sustained at 96 h. At 24 h, 1% and 0.3% ethanol decreased PGE 2 release by 88% and 75% respectively. This effect was sustained for 96 h only at the higher concentration. In the closed system, 0.1% ethanol increased TdR incorporation by 38% at 24 h. However, there was no effect on the labeling of CDP or NCP. Because its boiling point is 21 °C, acetaldehyde could only be tested in the closed system. Acetaldehyde markedly inhibited bone metabolism. At 24 h, 0.003% (0.54 mM) to 0.01% (1.79 mM) acetaldehyde caused a dose-related inhibition of TdR incorporation from 23 to 45%. At 0.01% and 0.03% acetaldehyde inhibited proline incorporation into CDP by 48% and 94% and NCP by 40% and 74% respectively. In the resorption system, acetaldehyde at 0.01% inhibited PTH stimulated bone resorption, while ethanol (0.1 to 1.0%) had no effect. We conclude that ethanol has little effect on bone formation or resorption; however, the metabolite acetaldehyde is a potent inhibitor of bone formation and resorption. These results may be of significance in the development of osteoporosis associated with chronic alcohol use.