The in vitro effect of the nonsteroidal antiestrogens tamoxifen (TAM) and clomiphene (CLO) on bone resorption was investigated. TAM (100 microM) and CLO (100 microM) completely blocked PTH (2 nM)-induced resorption; 10 microM TAM was ineffective in blocking resorption, while 40-50 microM partially inhibited the response. TAM (100 microM) also completely blocked prostaglandin E2 (30 nM)- and 1,25-dihydroxy vitamin D3 (0.5 nM)-induced bone resorption. A 16-h pretreatment with TAM blocked subsequent stimulation of resorption by PTH, whereas 3.5- or 7-h pretreatment with antiestrogen was ineffective in blocking the response. Both protein and DNA syntheses were inhibited by continuous treatment (48 h) with the antiestrogens. When antiestrogen-pretreated (16 h) bones were transferred to fresh medium not containing antiestrogen, protein and DNA syntheses recovered to approximately half the control (nonantiestrogen-treated) values within 48 h. Bone resorption, however, was still completely inhibited even though macromolecular synthesis had substantially recovered. Thus, mechanisms other than macromolecular synthesis inhibition could be involved in the inhibition of bone resorption by the nonsteroidal antiestrogens TAM and CLO.