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Changes in extrinsic fluorescence intensity, associated with step changes in membrane potential, have been studied in intracellularly or extracellularly stained squid axons, and in lipid bilayers, using six different aminonaphthalene dyes: 1,8-TNS; 2,6-TNS; 1,8-MANS; 2,6-MANS; 2,6-ANS and NPN. In all preparations the optical signals were found to be roughly proportional to the voltage applied. All signals had a very fast initial component, which was followed in some case by a slower change in the same direction. The slow component was observed only in intracellularly stained axons, and not for all chromophores studied. 1,8-TNS, 1,8-MANS and 2,6-MANS yielded the largest fluorescence signals in all preparations. The sign of these signals was independent of the type of membrane studied. However, the fluorescence changes of 2,6-MANS were opposite to those of 1,8-TNS and 1,8 MANS. Staining of both sides of the axolemma with 1,8-MANS or 2,6-MANS showed that these dyes yield larger signals when applied to the extracellular face. The changes in fluorescence light intensity of 2,6-TNS, 2,6-ANS and NPN were smaller and their sign depended on the membrane preparation studied. The comparison of the extrinsic fluorescence signals from the nerve membrane and the phosphatidylcholine bilayer suggests strong similarities between the basic structures of the two systems. The variety of observed signals cannot be easily interpreted in terms of changes in membrane structure. A possible alternative interpretation in terms of electrically induced displacements, rotations and changes in partition coefficient of bound chromophores, is discussed. [ABSTRACT FROM AUTHOR]