Leaflet tissue specimens prepared from porcine aortic valves treated with glutaraldehyde at low and high pressures have been subjected to 0.45 x 10(9) accelerated fatigue cycles in Ringer's solution. The waveform or crimped property of the collagen fibres in the leaflet tissue is an essential requirement for its ability to resist localized deformation during repeated flexure. High pressure glutaraldehyde fixation of the whole valve eliminated the crimp structure and resulted in the formation of localized kink sites in the tissue specimen during repeated flexure. Eleven separate sites of serious tissue disruption were observed in the three fatigue specimens obtained from the high pressure-treated valve. In contrast to this only one site of serious disruption was observed in the three fatigue specimens obtained from the low pressure-treated valve. Here fixation preserved the fully crimped morphology of the collagen. It is expected that the long-term mechanical durability of glutaraldehyde treated aortic valves can be substantially increased if careful consideration is given to the pressure at which initial fixation is carried out.