Protein synthesis initiation on prokaryotic mRNAs involves base-pairing of a site preceding the initiation codon with the 3' terminal sequence of 16 S rRNA. It has been suggested that a similar situation may prevail in eukaryotic mRNAs. This suggestion is not based on experiments, but on observation of complementarities between mRNA 5' noncoding sequences and a conserved sequence near the 18 S rRNA 3' terminus. The hypothesis can be evaluated by comparing the number of potential binding sites found in the 5' noncoding sequences with the number of such sites expected to occur by chance. A method for computing this number is presented. The 5' noncoding sequences contain more binding sites than expected for a random RNA chain, but the same is true for 3' noncoding sequences. The effect can be traced to a clustering of purines and pyrimidines, common to noncoding sequences. In conclusion, a close inspection of the available mRNA sequences does not reveal any indication of a specific base-pairing ability between their 5' noncoding segments and the 18 S rRNA 3' terminus.