Experimental studies on the biodistribution and placental transfer of in vivo metabolic constituents in pregnant rats were investigated with positron-emitting compounds such as 18F-2-fluoro-2-deoxyglucose, a mixture of 11C-glucose and 11C-fructose, 11C-L-methionine, 11C-D,L-leucine, 11C-adenine, 18F-5-fluoro-2'-deoxyuridine, 11C-S-adenosyl-L-methionine and 11C-coenzyme Q10. Sugars and amino acids transferred easily through the placenta and distributed into the fetal tissues to a similar extent as into the maternal tissues. The fetus-to-placenta ratios for the amino acids were always over 1.0, which indicated active transport of the amino acids in the placenta. On the contrary, other compounds have some limitation to transfer through the placenta in comparison with the sugars and amino acids. The fetus-to-placenta ratios of 11C-adenine, 11C-S-adenosyl-L-methionine and 11C-coenzyme Q10 were much less than the placenta-to-blood ratios, which indicated the presence of a certain placental barrier against these compounds. 11C-S-adenosyl-L-methionine and 11C-coenzyme Q10 were incorporated more into the fetal brain than into the maternal brain which is probably due to the nutritional requirement of the fetal brain and/or an immature blood-brain barrier.