The behaviour of lipid lamellae upon an applied hydrostatic pressure in different phase states is considered. We analyse the phase transitions from the bilayer phase states L-alpha or L-beta, to the interdigitated phase LbetaI. By considering separately the energies of chains and polar heads we obtain the expressions for the chemical potential of each state. The criteria for the existence of L-alpha or L-beta, states upon an applied pressure are derived. We conclude that the steeper increase of internal tension in the region of lipid heads in these states in comparison with the LbetaI one is the main physical cause of phase transitions to interdigitated phase upon an applied pressure. (C) 2002 Published by Elsevier Science B.V.