Expression of the rat insulin-like growth factor I (IGF-I) gene results in a number of mature mRNA species that differ in size primarily at the 3' end due to differential polyadenylation site usage. Additionally, alternate splicing in both 5' and 3' regions produces RNAs which have the capacity to encode different IGF-I precursor peptides. We have analyzed total and polysomal RNAs using Northern blot analyses and solution hybridization/RNase protection assays to assess the in vivo translatability of these various IGF-I mRNA species. The results suggest that all of the known splicing variants are found on polysomes and may, therefore, be translated into a number of IGF-I precursors in vivo. One particular 5'-untranslated (UTR) variant is relatively enriched in polysomal RNA, a finding which suggests that removal of some of the 5'-UTR sequences encoded by exon 1 may enhance translatability. Of the IGF-I mRNAs with different lengths of 3'-UTR, only the shorter species were found on polysomes, suggesting that some aspect of the long 3'-UTR may prevent translation. Thus, differential processing of the primary transcript of the IGF-I gene may serve to generate IGF-I mRNA species which specify different precursors as well as to control their relative translatability.