The fluorescence anisotropy probe perylene and the spin-labels 5-doxylsterate and 16-doxylstearate were used to estimate the order and internal microviscosity of the pigeon erythrocyte membrane upon perturbation by cationic or neutral amphipathic drugs (chlorpromazine, methochlorpromazine, tetracaine, and octanol) and an anionic drug, octanoic acid. Both methods gave identical results. The fluidity changes were found to strictly correlate with those of adenylate cyclase activity in the presence of GTP when perturbed by the drugs [Salesse, R., & Garnier, J. (1979) Biochim. Biophys. Acta 554, 102-113]. The cationic or neutral drugs, in an intermediate range of concentration, decreased the degree of organization and the internal microviscosity of the lipids together with the activity of the adenylate cyclase. At a higher concentration they reincreased them up to or higher than their initial level before the final destruction of the membrane structure and functions. This concentration effect was time dependent with tetracaine. The quaternary amine methochlorpromazine acted as chlorpromazine only on open ghosts. On intact cells, it inhibited catecholamine receptors at higher concentration and monotonously decreased the order and microviscosity, as the anionic amphipath octanoic acid did. This is taken as evidence that the inner leaflet of the bilayer is the seat for the observed multiphasic changes of viscosity and the control of adenylate cyclase and catecholamine receptors. This could stem from either a preferential intercalation or a surface effect of the amphipaths in the inner leaflet of the membrane. Since the basal activity of adenylate cyclase was not affected in the presence of drugs, it may be inferred that the enzyme holds its activity but that its stimulation is modulated by the membrane physical state.