The ultrastructure of the plasma membrane of mouse myocardial cells has been studied during development (from 10 days post-coïtum to the adult stage) using the freeze-cleave method, and the effect of glutaraldehyde-fixation on this structure has also been evaluated. Blebs, free of intramembranous particles (IMP), were found to be dependent upon the developmental stage and upon fixation with glutaraldehyde. On P and E fracture faces, the presence of IMP-free areas was age-dependent and fixation-independent. No difference was demonstrated in the IMP density during heart development. Fixation with glutaraldehyde retains in the plasma membrane approximately 40% of IMP, probably by preventing an alteration induced by glycerol. Whatever the developmental stage, E face-associated particles are significantly larger than P face-associated particles. The diameter of both types of particle increases slightly but irregularly as myocardial cells mature. Fixation reduces the particle size on both fracture faces in such a way that no significant difference between them can be demonstrated.