Ventricular pacing causes asynchronous electrical activation of the ventricular wall, because impulse conduction occurs via muscle fibers rather than via the Purkinje system. Chronic (up to 3 months) ventricular pacing caused about 30% decrease of wall mass in early activated regions but did not change wall mass in late activated regions. These are the first data indicating that chronic asynchronous activation induces asymmetrical structural adaptations. This asymmetry is likely to be evoked by regional differences in contractile work, as demonstrated in previous experiments from our laboratory. The nature of the structural adaptation as well as its clinical implications deserve more detailed investigation.