Absorption spectra measurements of isolated planarian ocelli by a microspectrophotometer (MSP) and intraocellar recordings of the early receptor potential (ERP) were carried out in order to characterize in situ planarian rhodopsin (pRh) and its photoproducts, The MSP spectra of the isolated ocelli revealed lambda(max) at about 500 nm, The ERP evoked by a test stimulus was a positive monophasic waveform in the dark but became negative during exposure to violet light. During subsequent darkness, the ERP rapidly reverted to a positive waveform but with a smaller amplitude than before exposure. The ERP amplitude recovered to its initial level upon exposure to red light, The ERP experiments suggest that pRh produces two metarhodopsin intermediates, with lambda(max) longer than that of pRh: the metastate responsible for the negative ERP converts to another metastate that results in a smaller ERP in the dark-adapted ocellus.