The metabolism of RNA has not been studied in the osteoclast (OC) because these bone-resorbing cells are only available in small numbers and cultures are always contaminated with other cells. Using two single-cell assay techniques, tritiated uridine (3H-UdR) autoradiography and gallocyanin quantitative cytophotometry, we have examined RNA synthesis in OCs isolated from neonatal rats. Oligo-nuclear OCs showed greater nuclear uptake of 3H-UdR than cells with many nuclei, and the variance of nuclear labeling within polykarya was greater in the latter, possibly because they contain nuclei of various ages. Salmon calcitonin (sCT) was a potent (ED50 approximately 5 x 10(-12) M) and rapid (40% reduction in 2 h, 75% reduction in 6 h) inhibitor of 3H-UdR uptake, and also reduced cytochemical total cellular RNA by 22% within 4 h. Forskolin (10(-5) M) inhibited nuclear uptake of 3H-UdR, suggesting that the sCT response may be mediated by cyclic AMP. Following a short (30 min) exposure to sCT, there was a progressive decline in labeling, followed by complete recovery by 4.5 h, a response possibly related to the phenomenon of calcitonin-induced persistent activation of adenylate cyclase. Inhibition of OC RNA synthesis may be an important component of its anti-resorptive action.