Large dense core vesicles (LDV) were purified from bovine splenic nerve homogenates by the sucrose-D2O density gradient method. Vesicles were subjected to a 50% increase and decrease in osmolarity from the control 330 mosmol 1(-1) by adjusting sucrose or potassium phosphate buffer during pre- and/or postfixation. Control vesicles with a mean diameter of 717 A readily swelled to approximately 1050 A and shrunk to approximately 600 A in the hypotonic and hypertonic media, respectively, with either sucrose or phosphate buffer. The dense core responded similarly but to a lesser degree. Prefixation in glutaraldehyde had little effect on vesicle sensitivity to subsequent tonicity change, not did the fixative per se exert an obvious osmotic effect. Thus, final vesicle size was largely determined by the OsO4 postfixation medium and principally by the vehicle rather than the fixative. In controls there was a mixture of spherical to oblate vesicles mostly filled with an electron-dense matrix. Upon swelling, more vesicles became spherical and nearly all had a prominent translucent halo between core and membrane. Upon shrinking, more vesicles became oblate, the halo was obliterated and the electron-density of the matrix increased. Frequency distributions of vesicle diameters at different tonicity clearly indicated that the diameter of LDV could overlap the 400-500 A range characteristic of small dense core vesicles; however, there was no suggestion of a population of the latter in the purified LDV fraction. Implications are discussed concerning the biochemical and morphological identification of 'light' and 'heavy' density peaks of noradrenaline and dopamine beta-hydroxylase from mixed vesicle populations and the possible relevance of changes in vesicle shape to a functional state in situ.