The locus of the structural gene encoding tyrosine hydroxylase,Th, the rate limiting enzyme for catecholamine biosynthesis, was mapped to the distal end of mouse Chromosome (Chr) 7. A DNA probe of genomic origin of rat tyrosine hydroxylase was used to detect restriction fragment length variants among 8 inbred mouse strains. The strain distribution pattern of 77i allelic variants in 3 sets of recombinant inbred mouse strains was determined. Comparison of the strain distribution patterns ofThalleles with those of previously typed loci suggestedThwas located on Chr 7. The Chr 7 assignment forThwas confirmed by analyzing 108 mice produced from an (NZB×SM)F1×NZB backcross. Moreover, theThlocus was positioned distally on Chr 7. Mouse Chr 7 and human Chr 1 lp (the location of the human tyrosine hydroxylase gene) are known to share several homologous loci. With the addition ofTh, the homology between the distal 2/3 of mouse Chr 7 and human Chr lip appears extensive.