&NA;Pressure‐volume relations were investigated on a model aneurysm wall made of elastic tissue and collagen. The model wall had a Young's Modulus of 2 × 107dynes/cm2, approximating the elastance of fresh aneurysm walls obtained at autopsy. The model wall was fixed over the top of a glass T‐tube, 6 mm in diameter. Pressure pulse waves of water or outdated human blood entered at the bottom of the T‐tube and exited by way of a controlled resistance, while pressure was monitored by a strain gauge and recorded on an ink writer from the other arm. Incremental increases in systolic pressure produced a nonlinear N‐shaped pressure‐volume curve. There was an initial linear enlargement in volume with increased pressure until a pressure threshold was reached at a point of high nonlinear compliance. At this point there was an abrupt jump in volume (mean increase = 70 ± 14%) to a new stable equilibrium volume. A further increase in pressure (24 ± 7 mm Hg) could eventually cause aneurysm rupture. This always occurred in the thinner, more compliant part of the wall. With higher pulse rates, the jump in volume occurred at a smaller volume, whereas with thinner aneurysm walls of higher compliance the pressure threshold for a jump in volume was significantly lower. At a higher peripheral resistance there was a higher pressure threshold. The authors suggest that there is one possible mechanism that may occur in the neck of some developing human aneurysms and could explain early and rapid initial growth in size. (Neurosurgery24:722‐730, 1989