A series of MLC2-chymase fusion genes consisting of nested 5' deletion of the chymase promoter region were constructed and transfected into primary cultured neonatal Wistar rat myocardial cells according to the modified calcium phosphate precipitation method. Total RNA Dot-blot hybridization and analysis of densitometric scanning demonstrated that MLC2 promoter is enough to direct human heart chymase gene transcription in cultured neonatal rat myocardial cells. The highest level of expression was found with the MLC2-chymase construct containing 6 bp of the chymase promoter region. More sequence of the chymase promoter region between MLC2 promoter and the ATG of chymase gene resulted in a subsequent decrease in the expression of human heart chymase gene.