We report the effects of pulsed electromagnetic fields (PEMFs) on the responsiveness of osteoclasts to cellular, hormonal, and ionic signals. Osteoclasts isolated from neonatal rat long bones were dispersed onto either slices of devitalised cortical bone (for the measurement of resorptive activity) or glass coverslips (for the determination of the cytosolic free Ca 2/ concentration, [Ca 2/ ]). Osteoclasts were also cocultured on bone with osteoblastlike, UMR-106 cells. Bone resorption was quantitated by scanning electron microscopy and computer-assisted morphometry. PEMF application to osteoblast-osteoclast cocultures for 18 hr resulted in a twofold stimulation of bone resorption. In contrast, resorption by isolated osteoclasts remained unchanged in the presence of PEMFs, suggesting that osteoblasts were necessary for the PEMF-induced resorption simulation seen in osteoblast-osteoclast cocultures. Furthermore, the potent inhibitory action of the hormone calcitonin on bone resorption was unaffected by PEMF application. However, PEMFs completely reversed another quite distinct action of calcitonin on the osteoclast: its potent inhibitory effect on the activation of the divalent cation-sensing (or Ca 2/ ) receptor. For these experiments, we made fura 2-based measurements of cytosolic [Ca 2/ ] in single osteoclasts in response to the application of a known Ca 2/ receptor agonist, Ni 2/ . We first confirmed that activation of the osteoclast Ca 2/ receptor by Ni 2/ (5 mM) resulted in a characteristic monophasic elevation of cytosolic [Ca 2/ ]. As shown previously, this response was attenuated strongly by calcitonin at concentrations between 0.03 and 3 nM but remained intact in response to PEMFs. PEMF application, however, prevented the inhibitory effect of calcitonin on Ni 2/ -induced cytosolic Ca 2/ elevation. This suggested that the fields disrupted the interaction between the calcitonin and Ca 2/ receptor systems. In conclusion, we have shown that electromagnetic fields stimulate bone resorption through an action on the osteoblast and, by abolishing the inhibitory effects of calcitonin, also restore the responsiveness of osteoclasts to divalent cations.