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Calcium concentrations in serum, cerebrospinal fluid (CSF), and perilymph were determined before and after the induction of a hypocalcemic state by a simultaneous injection of thyrocalcitonin and phosphate in the chinchilla. Prior to injection, the perilymph and CSF calcium concentrations were about equal and were approximately 70% of the serum calcium concentration. 1 h after the injection of thyrocalcitonin, serum calcium levels dropped by about one fourth and remained approximately stable for 3 h, whereas perilymph and CSF calcium concentrations showed no parallel decline. These results suggest the existence of mechanisms which are responsible for the maintenance of constant level of important cations in perilymph.