It is well established that osteoclasts use a vacuolar-type H(+)-ATPase (V-ATPase) for proton pumping during bone resorption and that specific V-ATPase inhibitors such as bafilomycin A1 abolish osteoclastic bone resorption in the bone slice assay. It has been reported that the V-ATPase in avian osteoclasts can be distinguished from the V-ATPase expressed in most other cells, by virtue of its inhibition by vanadate and nitrate ions. In order to determine whether the V-ATPase in mammalian osteoclasts can be similarly distinguished, we have investigated the effects of vanadate and nitrate on bone resorption by rat osteoclasts in the bone slice assay, in comparison with known V-ATPase inhibitors, bafilomycin A1 and WY 47766, that also inhibit the chicken osteoclast V-ATPase. The results indicate that, unlike the avian osteoclast V-ATPase, the mammalian osteoclast V-ATPase is pharmacologically similar to the V-ATPase in other cells.