Summary: A study was undertaken to determine whether, in antibiotic sterilised and stored valves, the state of preimplantation leaflet viability could be shown to influence valve structure and function following isotopic allotransplantation in dogs. Fourteen viable and 12 nonviable valves were assessed after periods of up to 8 weeks' implantation. Assessment of valve structure was made macroscopically with measurement of leaflet surface areas, and microscopically. Pressure measurements were made across the allografted valve both at insertion and at removal. The results of this study indicate that preimplantation viability results in gross valve leaflet distortion and shrinkage with consequent loss of function. Nonviable valves, in contrast, showed minimal alteration in valve dimensions with retention of normal function. These findings have considerable implications in the preparation and clinical use of allograft heart valves.