Enhanced hydrostatic pressure can induce phase transitions in hydrated lipid bilayers especially those composed of saturated phospholipids. In this work the phase behavior of fully hydrated DPPC (1,2-dipalmitoyl-sn-glycero-3-phosphocholine) and POPC (2-oleoyl-1-palmitoyl-sn-glycero-3-phosphocholine) bilayers as a function of pressure up to 3000 atm has been examined in atomic detail on time scales of up to 1.0 mu s, using the molecular dynamics simulation technique. DPPC bilayers formed a rippled gel-like phase comprising a minor disordered fluid-like region and a major ordered gel-like region at 1000 atm, a partially interdigitated gel like phase at 2000 atm, and a gel like phase with most of the lipid acyl chains tilted with respect to the plane of the bilayer at 3000 atm. POPC bilayers formed a rippled gel-like phase at 1800, 2400, and 3000 atm. The phase behavior observed for both DPPC and POPC bilayrs is in agreement with experiment. The simulations provide insight into the structural changes of DPPC and POPC bilayers as a function of pressure and demonstrate the ability to model biologically relevant lipid systems under high hydrostatic pressure.