Myosin isotype composition was examined in ventricular infundibular muscle from 40 patients with tetralogy of Fallot, aged from 7 months to 38 years. Results were compared with normal samples of ventricular infundibulum from subjects in the same age range, from 18-20 week old fetuses to 6 month old neonates, and from 43-81 year old adults. Myosin light chain isotypes were examined by one dimensional and two dimensional gel electrophoresis and quantified densitometrically. Heavy chain isotypes were examined by electrophoresis of whole heavy chains and peptide mapping after limited proteolytic digestion with chymotrypsin. At mid-gestation in normal tissues, only ventricular light chain 2 was present but light chain 1 consisted almost equally of atrial and ventricular isotypes. Amounts of atrial light chain 1 declined towards birth and disappeared during the first year after birth, gradually being replaced by ventricular light chain 1. Relative amounts of total light chains 1 and 2 remained equal. In tetralogy of Fallot atrial light chain 1 expression did not cease neonatally with mean values of 11.8% of total light chain 1 present between 7 months and 2 years, decreasing to 1.7% at 6.5-12 years and then increasing again to 3.4% in adults. A value of 34% atrial light chain 1 was present in one subject. As in normal subjects, equimolar amounts of total light chains 1 and 2 were retained. No evidence of new light chain isotypes was found in tetralogy of Fallot. Heavy chain expression was constant in normal infundibulum with only beta-heavy chain (V3 isozyme) present in the fetus, neonate, and adult.(ABSTRACT TRUNCATED AT 250 WORDS)