Conceptus uptake of the 106RuNO-nitro complex in relation to gestational stages

106RuNO-nitro complex was administered intravenously to pregnant rats to study uptake of the radioruthenium complex by the conceptus in relation to gestational stages. Each conceptus was sampled periodically with respect to its placenta, fetal membrane, fetal fluid and fetus. Perceptible radioactivity in the fetus was detected only in the later stage of gestation and its relative concentration, defined as the ratio radioactivity per unit weight in the body tissue at sacrifice to that in the whole-body at dosing, was very low compared with other tissues. The average number of fetuses in one litter was 13 and the transfer rate of nitro complex into the fetuses 24 hr after injection to rats on the 20th day of gestation was about 1% of initial maternal dose. The relative concentration in the placenta and fetal membrane was much higher than in the fetus and decreased with time after injection. These results indicate that the placenta and fetal membrane play significant roles as barriers to the transfer of 106RuNO-nitro complex into the fetus.