The effect of the calcium antagonist verapamil on 1 alpha-hydroxy-vitamin D3 (1 alpha (OH)D3) stimulated bone resorption in tissue culture has been investigated. It was found that verapamil in concentrations above 20 mumol/l reduced 1 alpha (OH)D3-stimulated mineral mobilization, as measured by release of in vivo incorporated 45Ca from mouse calvarial bones. The inhibition of verapamil could be seen already 3 h after exposure to the drug. The increased degradation by 1 alpha (OH)D3 of the organic matrix in the calvaria, as assessed by the release of in vivo 3H-proline labelled collagen, was also reduced by verapamil. The inhibitory effect of the drug on 45Ca release was reversible after withdrawal. 1 alpha (OH)D3 increased the release of stable calcium and beta-glucuronidase, and these effects could be blocked by verapamil. Increasing medium calcium concentration from 1.8 to 5 mmol/l slightly reduced the inhibitory capacity of 50 mumol/l verapamil on 1 alpha (OH)D3-stimulated 45Ca release. These data indicate that stimulation of osteoclasts by hydroxylated metabolites of vitamin D to resorb bone and secrete lysosomal enzyme is dependent on an increased availability of free intracellular calcium.