A Moiré fringe method was used to determine the shape of inflated disks of fresh bovine pericardium. The results suggested that the tissue was anisotropic and allowed the identification of the directions of the axes of elastic symmetry. Quantitative biaxial inflation and simple tension tests confirmed the anisotropy. In the circumferential direction the tissue was more extensible and had greater mean strength (18 MPa against 2.5 MPa) and mean terminal modulus (46 MPa against 14 MPa) than in the root-to-apex direction. The tissue was also optically anisotropic when viewed by transmitted polarised light and the directions of mechanical and optical anisotropy were related.