To study the potential role of insulin-like growth factor-I (IGF-I) during early embryogenesis, we have used the amphibian Xenopus laevis, a versatile model of vertebrate development. Using polymerase chain reaction (PCR)-based cloning strategies, we have previously identified two different nonallelic Xenopus IGF-I genes (IGF-I' and IGF-I"). Both are expressed in similar quantities in adult liver. We now report the use of a modification of the reverse transcription-PCR method, designated RNA template-specific PCR (RS-PCR), to detect IGF-I messenger RNA (mRNA) from single oocytes and embryos. The same primer pair was used to amplify both IGF-I' and IGF-I" mRNAs. Slot blot analysis of the RS-PCR products with internal oligonucleotide probes that specifically recognize IGF-I' or IGF-I" sequences revealed that only IGF-I' mRNA was present in follicles surrounding mature (stage VI) oocytes; neither IGF-I mRNA was present in follicles surrounding less mature oocytes (stages I and IV) or within oocytes or unfertilized eggs. After fertilization, IGF-I' mRNA was first detected during early organogenesis (stages 21-23), and increased during subsequent stages of development. To localize early IGF-I' expression, a stage 27 embryo was sliced into 24-microns cross-sections; RS-PCR and slot blot analysis were performed on RNA extracts from consecutive sections. IGF-I' mRNA was expressed in all sections, but was most abundant in the body region from which the visceral organs were developing. In contrast, to the early expression of IGF-I', IGF-I" mRNA was not detected until stage 41, a period corresponding to premetamorphic growth. Reverse PCR for Xenopus GH mRNA demonstrated that the onset of GH gene expression was coincident with the onset of IGF-I" gene expression (stage 41). These data suggest that the early expression of IGF-I' may be GH independent, whereas later expression of IGF-I" is GH dependent. We conclude that the two nonallelic IGF-I genes are expressed differentially in a stage-specific manner and suggest that IGF-I' may play an important role during early organogenesis, whereas IGF-I" may be important for premetamorphic growth.