Strain energy release rates have been calculated by Finite Element Analysis (FEA) for growth of a small crack in a cylindrical rubber block under tension. The crack was assumed to lie in the center of the block in a plane perpendicular to its axis. The rubber was taken to be virtually incompressible in bulk and non-linearly elastic. Only small changes were found in the stress-concentrating effect of the crack for imposed tensile strains of up to 150 per cent. Secondly, the rubber cylinder was assumed to be bonded between two parallel flat rigid plates. The stress-concentrating effect of the crack was found to increase markedly as the thickness of the bonded rubber block was reduced, but again it was only slightly dependent on the applied tensile strain.