The in vivo synthesis rates of myosin isozyme heavy chainsβandαwere measured in right ventricular (RV) muscle at 2 and 4 days following pulmonary artery constriction in rabbits, together with measurements of their relative mRNA levels. The synthesis rate ofβ-myosin heavy chains was elevated in 2-day (0.27 ± 0.06 day−1or 2.5 ± 0.7 mg/g RV/day, mean ± SD) and in 4-day (0.25 ± 0.08 day−1or 2.8 ± 1.0 mg/g RV/day) pressure overload, when compared to untreated rabbits (0.15 ± 0.04 day−1or 1.5 ± 0.4 mg/g RV/day). However, the synthesis rates ofα-myosin heavy chains in the same hearts were not altered significantly. There was a differential increase in the fractional synthesis rate ofβvs.αheavy chains in 2-day and 4-day pressure overload and in 2-day shams, suggesting switching towardβheavy chain synthesis had occurred at these time points.βheavy chain synthesis, as a proportion of total (α+β) heavy chain synthesis, was significantly higher in 4-day pressure overload (78 ± 9%) than in 4-day sham rabbits (63 ± 6%). This increase in relativeβ-synthesis was associated with a significant increase in the relative proportion ofβheavy chain mRNA level (76 ± 13% vs. 56 ± 7%). Furthermore, relativeβ-synthesis and theβ-mRNA levels correlated linearly with each other in all experimental groups. We conclude that during the early stages of pressure overload 1) the synthesis rate ofβ-myosin heavy chains is accelerated without a reciprocal decrease in a-myosin heavy chain synthesis, and 2) an increase inβ-myosin heavy chain expression appears to be achieved mainly by modulation of pretranslational events.