The mRNAs which code for the fusion proteins of the morbilliviruses (measles virus, canine distemper virus, and rinderpest virus) have unusually long 5′ untranslated regions (UTRs) which are GC-rich and are capable of folding into extensive secondary structures. In measles virus the first AUG codons in the fusion (F) protein mRNA are in close proximity at nucleotide positions 574 and 583 and protein translation is initiated at the second position. In the canine distemper virus (CDV) and rinderpest virus (RPV) F gene transcripts the analogous initiation codons are preceded by several other AUG codons many nucleotides upstream either in the same reading frame or at the beginning of other short open reading frames. We have studied the effect of deleting these upstream regions on the production of the fusion proteins of both CDV and RPV from cDNA constructs. Within the cells the presence of these regions enhances the production of the F protein while, in contrast, the production of the authentic F protein from in vitro translations using RNA transcripts is inhibited by these sequences.