Using synchrotron radiation as the excitation light, we studied the fluorescence parameters of perylene incubated with pigeon erythrocyte membranes and with an isotropic viscous medium, the Primol 342 oil. From 4 to 37 degrees C, we observed a single lifetime of 4.5 ns in the oil and two with the membrane (tau 1 = 1-1.4 ns and tau 2 = 5.4-6.1 ns). The dependence upon temperature of the rotation correlation time of perylene (theta) in the oil was characteristic of an isotropic medium, whereas the limiting value of anitropy (r infinity) was zero. With the membrane, r infinity decreased from 0.14 to 0.06 and theta from 2.9 to 0.5 ns, indicating a greater amplitude and frequency of molecular motions. The addition of chlorpromazine, indomethacine, tetracaine, n-octylamine, octanol or octanoic acid to the membrane decreased the tau 1 and tau 2 values. This would stem from the disorganization of the membrane induced by the drugs.