Computer analysis of nucleotide sequences of 5'-untranslated regions (5'-UTR) of higher plants mRNA adopted from the EMBL nucleotide sequence databank was carried out. It was demonstrated that the average nucleotide frequencies of the leader sequences and adjacent regions of basal promoters are similar, whereas introns and 3'-UTR have a higher content of T and a lower content of C. A particular 5'-UTR contextual feature is a misbalance in the content of complementary nucleotides; probably a stable secondary structure adversely affects the translation properties of the leader sequence. About 20% of 5'-UTR contain AUG triplets, which is twice the earlier estimate. Considered are the properties of leader open reading frames (uORF), the possible causes of their high content in 5'-UTRs of eukaryotic mTNAs, and correlations between the features of uORFs and of the protein-encoding sequence of the gene. It is demonstrated that in effectively translated mRNAs the leader AUG triplets are more frequently located in a nonoptimal context, whereas terminating codons of uORFs more frequently exist in the optimal one. A hypothesis is put forward that the efficiency of termination at the uORF stop codon might substantially interfere with the mRNA translation activity.