The fluidity of the rat erythrocyte membrane was evaluated by measurement of excimer fluorescence of an intra-molecular forming fluorophore, 1,3-di(1-pyrenyl)propane. The polyene ionophoric antibiotics, amphotericin B and nystatin, were found to fluidize the erythrocyte membrane, as assessed by the increase in the excimer/monomer fluorescence intensity ratio, by 42 and 13%, respectively, compared with control samples. In contrast, of the peptide ionophoric antibiotics, valinomycin demonstrated about twice the effect which gramicidin A had on depressing the fluidity of the erythrocyte membrane. On the other hand, the general lipophilic anaesthetics, propanidid and althesin, led to an increase, by 70 and 32%, respectively, while the local anaesthetic, procaine, led to a decrease by 20%, in the fluidity of the erythrocyte membrane. These results were explained in the light of the partition coefficients determined for these drugs in decane and native membranes, their affinities for specific membrane components and the changes which they induce in the permeability properties of erythrocyte and other biological membranes.