We have examined the tissue distribution and developmental regulation of two low molecular weight cytosolic fatty acid binding proteins. Based on their initial site of isolation, they have been referred to as liver and intestinal fatty acid binding proteins (FABP). Cloned cDNAs were used to probe blots of RNAs extracted from a wide variety of adult rat tissues as well as small intestine and liver RNA obtained from fetal, suckling, and weaning animals. The highest concentrations of “liver” FABP mRNA were found in small intestine and liver. “Intestinal” FABP mRNA is most abundant in small bowel RNA while only trace amounts were encountered in liver. Both mRNAs were detectable in stomach, colon, pancreas, spleen, lung, heart, testes, adrenal, and brain RNA at 1-8% the concentrations observed in small intestine. Accumulation of both mRNAs in the small intestinal epithelium increases during development. The mRNAs are first detectable between the 19th and 21st day of gestation. They undergo a coordinated 3-4-fold increase in concentration within the first 24 h after birth. Thereafter, gut levels of intestinal FABP mRNA remain constant during the suckling period while liver FABP mRNA increases an additional 2-fold. Liver FABP mRNA levels are also induced in hepatocytes during the first postnatal day but subsequently do not change during the suckling and weaning phase, despite marked alterations in hepatic fatty acid metabolism. These observations support the concept that the major role of these proteins is to facilitate the entry of lipids into cells and/or their subsequent intracellular transport and compartmentalization. The data also raise questions about the identity of extragastrointestinal FABPs.