The sequence of intermediates in the hR photocycle and their interconversions have been the subject of numerous studies (Va´ró et al., 1995a, b; Hazemoto et al., 1984; Zimányi and Lanyi, 1989a, b; Zima´nyi et al., 1989; Spencer and Dewey, 1990; Ames et al., 1992; Dioumaev and Braiman, 1997a).