Abstract Rod outer segment membranes were studied by means of the spin-label technique. The spin label 3-maleimido-2,2,5,5 tetramethyl-1-pyrrolidinyloxyl was shown to be mainly attached to sulphydryl groups of the membrane protein. Solubilization of rhodopsin by Triton X-100 detergent produced a significant modification of the spin resonance spectra, which is interpreted as resulting from a conformational change occurring in the lipid-protein complex. When the rhodopsin molecule was embedded in the membrane layer, a correlation time larger than 2.5·10 −7s was measured. In Triton X-100 solution, a correlation time of 7·10 −8s characterizes the macromolecule.