Electrophysiological and biochemical aspects of polarity determination and morphogenesis were studied in regenerating Acetabularia. The Ca(++), Mg(++) ionophore, A23187, reversibly inhibits the formation of apical structures (whorls and caps) but does not arrest longitudinal growth. This normal growth correlates with normal electrophysiology as reflected in an apico-basal electrical potential gradient and spontaneous recurrent action potentials which propagate from apex to base. However, the ionophore markedly elevates (32)PO 3 (3-) incorporation into the cortical cytoplasm which is normally low apically and rises to a maximum at the base. A molecular model of membrane-dependent morphogenesis is suggested.