The plasma and synovial fluid concentrations of synthetic salmon calcitonin in 10 patients with knee joint effusions have been compared after a single i.v. dose of 200 I.U. calcitonin. Plasma and synovial fluid concentrations of calcitonin were measured using a specific RIA before and 30 and 60 min after administration. Calcitonin was not detectable at zero time in plasma or in synovial fluid. Plasma calcitonin concentrations 60 min after administration were significantly lower than at 30 min, while the synovial fluid concentration remained relatively constant. The results show that synthetic salmon calcitonin penetrates into the articular cavity after a single i.v. dose of 200 I.U. and that a steady concentration persists there over 60 min.