The postoperative course of 27 thyroparathyroidectomized dogs is described with periods of observation up to 600 days. Evidence obtained at necropsy and/or reoperation supports the idea that most, if not all, of the animals had been rendered aparathyreotic. Clinically the animals appeared to separate into one group of 18 which demonstrated severe continuing tetany and another group of 9 which either never demonstrated frank tetany or else did so only transiently.When the results of the serum calcium and inorganic phosphorus analyses available for these animals were analyzed according to this grouping of symptomatic and asymptomatic dogs, statistically significant differences were found between the 2 groups. While the differences might be explained in several ways, some evidence is presented suggesting that differences in intestinal absorption of dietary calcium may be one factor. (Endocrinology 76: 9, 1965)