The localization of glial cell line-derived neurotrophic factor (GDNF) mRNA was studied by in situ hybridization in rat from embryonic (E) day E10 to E15. At E10, GDNF mRNA is found in the urogenital field and the cranial part of the gut. At E11, the most abundant expression of GDNF mRNA is seen in the epithelial cells of the second, third and fourth pharyngeal pouches, the third and fourth pharyngeal arches and pharynx. Also mesenchymal cells of the gut and mesonephric tubules contain GDNF mRNA. At E13, expression is observed in the mesenchymal cell layers of the oesophagus, intestine and stomach, the mesenchymal cells around the condensing cartilages and metanephric kidney mesenchyme. Also, the epithelia of Rathke's pouch and pharynx are intensely labelled. High expression of GDNF mRNA continues at E15 in kidney, gastrointestinal tract and cartilage. At that stage, GDNF mRNA is seen also in whisker pad and skeletal muscles. The distribution of GDNF mRNA in embryonic rat suggests important roles for GDNF in the early differentiation of the kidney tubules, the innervation of the gastrointestinal tract and the differentiation process of the cartilage and muscle. Our results indicate novel functions for GDNF outside the nervous system.