Recovery from bright light was studied in macaque rods by measuring the membrane current of single outer segments. The recovery phase of some responses displayed a plateau current of about one picoampere lasting for several seconds. The following evidence suggests these "steps" are single photon responses of abnormally long duration. (1) Over a limited range of intensities, step amplitude remained constant and summed linearly with intensity. The collecting area for step generation was about 2.6 x 10(-3) microns2. (2) Step duration varied exponentially with a mean duration of about 6.5 s. (3) Fluctuation analysis of the tail currents was consistent with the idea that a step is evoked by isomerization of a single rhodopsin molecule, and that only 1 in 400 isomerizations leads to a responses with a step-like waveform. (4) With only the distal portion of the outer segment in the electrode, the polarity of the step response reversed when the proximal portion of the outer segment was illuminated, indicating that step generation results from a local change in outer segment conductance near the site of photon absorption. (5) The probability of eliciting a step varied with the wavelength of light in the manner expected from the absorption spectrum of rhodopsin.