The insulin-like growth factor II (IGF-II) gene generates multiple mature transcripts with different 5' untranslated regions (5'UTR) but identical coding regions and 3'UTRs. We have analysed the translational regulation and decay of the transcripts. Human IGF-II mRNAs provide a provocative example of translational discrimination in which only the minor 4.8 kb mRNA is actively engaged in protein synthesis while the major 6.0 kb mRNA is present in a 100S RNP particle. The 6.0 kb mRNA exhibits a structured 5'UTR of 1170 nucleotides that function as a cis-acting translational attenuator. IGF-II transcripts are processed by endonucleolytic cleavage 1209 and 2183 nucleotides downstream from the translation termination codons in the rat and human, respectively. The cleavage site is situated in a highly conserved and structured domain that exhibits two large hairpins and an intramolecular guanosine quadruplex. The structural elements may provide binding sites for trans-acting factors and ensure that the cleavage site is not sequestered in stable RNA structures. Since expression of IGF-II is initiated from minimal promoters and finished by constitutive secretion from the cell, regulatory events governing IGF-II production are likely to be implemented between the transcriptional and the posttranslational level. The combined effect of translational discrimination and endonucleolysis may therefore play an important role in the regulation of IGF-II expression.