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The LD50 values for calcium chloride in F3 adult male and female rats raised from mothers parathyroidectomized on the 15th day of pregnancy (l5d-PTx) were much lower than in normal adult rats. Parathyroidectomy on the 5th day of pregnancy (5d-PTX) failed to induce the hypersensitivity to a lethal dose of calcium in the F3 rats. Despite the low LD50 values for calcium chloride in the 15d-PTx F3 rats, their serum calcium level at 1.5-2 min after the injection of calcium solution did not differ from the levels in normal and 5Cl-PTX F3 rats. KEY WORDS calcium hypersensitivity / LD50 / offspring/ parathyroidectomyWhen the parathyroid glands of pregnant rats were removed on the 5th day of gestation, the first generation rats born to these mothers exhibited a less marked decline in serum calcium levels following parathyroidectomy (PTX) than in normal controls and this characteristic feature was transmitted down to the 3rd or 4th generation rats (3). We report here that the LD,-,0 for calcium chloride solution in the F3 adult rats raised from mothers parathyroidectomized on the 15th day of gestation was much lower than that obtained in the rats raised from normal mothers or from mothers parathyroidectomized on the 5th day of gestation.Adult male and female (SPF) rats of the inbred Wistar-Imamichi strain (Animal Breeding Laboratory, Ohmiya, Saitama Prefecture) were mated. The day on which sperms were present in the vaginal smears was designated as day 0 of pregnancy. The parathyroid glands were surgically removed on the 5th (5Cl-PTX) or 15th (l5Cl-PTX) day of gestation under ether anesthesia. The mothers were given 1% calcium lactate solution as drinking water for 3 days before parturition and for the first 3 days of lactation. When calcium was not given they died of tetanic seizures on the 21st or 22nd day of pregnancy. Their infants were weaned at day 21 after birth and mated by brother-sister mating when 3 months old to develop subsequent generations. The animals were supplied with commercial stock diet (calcium, 1.

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