The effect of thyrocalcitonin (TCT) on the calcium content of liver slices has been studied. At 30 min incubation period, TCT significantly (p<0.01) increased the accumulation of calcium in the concentration range from 0.1 to 8.0 mU/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 calcium accumulation was additively enhanced. On the other hand, the radiocalcium (⁴⁵CaCl₃) 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). These data suggest that TCT increased calcium accumulation in the liver cells by inhibiting the efflux of calcium.