In the rat, 1 week of a calcium-deficient diet will recruit large numbers of osteoclasts to long bone endosteal surfaces. Subsequent calcium supplementation causes the osteoclasts to disappear in 1-3 days but as little as 3 hours of calcium supplementation reduces the extent of their ruffled borders. To test the hypothesis that at some point there is an irreversible inhibition of osteoclasts, male, weanling, calcium-deficient, SD rats were given various amounts of calcium-containing diet followed by a 12-hour fast. No changes in seven morphological indices of osteoclast activity were found. The hypothesis that the fast had reversed the effects of the calcium diet was supported by a second experiment indicating that no inhibition threshold had been reached. Another experiment showed differences in the degree of osteoclast inhibition with different amounts of calcium supplementation, again without evidence of a threshold. These experiments raised two questions: (1) Will fasting recruit osteoclasts in calcium-replete rats? and (2) Is osteoclast recruitment facilitated by the presence of postosteoclasts? The results of experiments testing these hypotheses support the conclusion that fasting and calcium deficiency maintain plasma calcium levels by different mechanisms, and post-osteoclasts are not available for reactivation. It is concluded that inhibition of osteoclasts by dietary calcium is a graded phenomenon, and when osteoclasts have lost contact with the bone surface they are unavailable for reactivation; a threshold has been reached.