The deformation of end-linked polymer networks was investigated by Monte Carlo simulations using the three-dimensional bond fluctuation model. A novel methodology is presented for constant pressure simulations on a lattice that is based on placing a sample between walls. Uniaxial expansion or contraction is performed in a number of intermediate steps by adjusting the wall-polymer potential. The network P-V behavior is found to be similar to the athermal linear chain P-V behavior at high volume fractions, but the network data deviate from that of the linear chains at low volume fractions. At large deformations, elastic chain dimensions are non-Gaussian in the direction of deformation, while remaining approximately Gaussian in directions perpendicular to deformation. Also, the chain segment orientation of the elastic chains in the network and linear probe chains that are trapped in undeformed and deformed networks was determined. The results suggest that segments near the ends of elastic chains in the network deform to greater extents than the chain segments in the center of the chains.