At 30 min incubation period, TCT [thyrocalcitonin] significantly (P < 0.01) increased the accumulation of Ca in the concentration range from 0.1-8.0 mU[units]/ml with half-maximal concentration occurring at about 0.1 mU/ml. During the 60 min incubation period, the effect of TCT on the liver Ca accumulation was additively enhanced. The radiocalcium (45CaCl2) efflux in the absence of the hormone increased linearly during the incubation period. The radiocalcium efflux in the presence of the hormone increased later with time than that in the absence of the hormone, and the inhibition of efflux was 25% of the control (P < 0.01). TCT possibly increased Ca accumulation in the liver cells by inhibiting the efflux of Ca.