Pregnant Sprague-Dawley rats were injected intravenously with [14 C]thymidine on day 13.5 of gestation, and the concentrations and radiation doses of14 C in the fetal brain and liver were determined by liquid scintillation counting and autoradiography with imaging plates. The concentrations of14 C in the whole fetal brains determined by liquid scintillation counting were 1.01% of the injected dose per gram wet weight at 6 h after injection and decreased to$0.39\%\ {\rm g}^{-1}$ at 48 h after injection. A significant accumulation of14 C was observed in the fetal liver: 3.8 and 0.51% of the injected dose per gram wet weight at 6 and 48 h after injection, respectively. Autoradiography showed that, especially at earlier periods after injection, there was remarkable concentration of14 C in the ventricular zone of the brain and the central region of the liver. With increasing time after injection, the distribution of14 C became relatively uniform. The concentrations of14 C in the ventricular zone of the fetal brain, determined by autoradiography, were much higher than those in the whole brain as determined by liquid scintillation counting. Cumulative radiation doses for 6-48 h after injection were 1.27 mGy for the whole fetus and 1.45 mGy for the whole brain. In contrast, the cumulative radiation dose for the ventricular zone of the brain which was determined by autoradiography was approximately 2.2 times that for the whole brain.

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