The signals which direct the excision of introns from eukaryotic pre-mRNA are not yet well understood. In order to define the signals for 5'-splice sites of mRNA splicing, nucleotide sequences including 5'-splice junctions of mammalian pre-mRNAs are analysed by means of syntactic pattern analysis. Taking this approach, we infer the grammatical rules which specify 5'-splice sites and construct a finite automaton which is the recognizer of the nucleotide sequences at 5'-splice sites. By scanning the automaton along nucleotide sequences, we can identify the positions of 5'-splice junctions with a degree of discrimination of up to 94-97% in the known genes, while the degree of prediction is in the range 50-55% in new genes.