Using early receptor potential (ERP) measurements, we show that the bistable pigment in the barnacle photoreceptor behaves according to the conclusions of the preceding article (Hochstein et al., 1978): (1) The populations of both stable states approach their steady-state or “saturation” values under steady illumination exponentially with the same rate constant; the wavelength dependence of this rate constant is called the relaxation spectrum. — (2) The saturation values are independent of initial population and of light intensity; the wavelength dependence of the saturation population is called the saturation spectrum. — (3) The measured relaxation and saturation spectra agree with those calculated, by the theory of the preceding article, from the experimentally determined transition parameters of the pigment system. — We then demonstrate the applicability of relaxation and saturation measurements to the question of whether a single bi-stable pigment system serves, or two or more separate systems serve, as the origins(s) of the ERP and of other phenomena observed in the barnacle photoreceptor: The prolonged depolarizing afterpotential (PDA) and its depression and prevention (anti-PDA). By showing that the relaxation spectra for these phenomena match one another and that of the ERP, and that the same is true for the saturation spectra, we demonstrate that these phenomena originate from the same single bi-stable pigment system as the ERP.