Changes in rhesus spectral sensitivity were induced at low-level (2.1 log td) repetitive exposure to a visible (514 nm) coherent source. Irradiation involved the entire retina. Long-term suppression of intermediate cones was evident in spectral sensitivity functions measured for a very fine photopic (1.85 min-1) acuity criterion. At slightly coarser acuity criteria (1.42 min-1) but still photopic, evidence of rod intrusion was obtained. Recovery from these effects was not obtainable 14 months post-exposure. Possible morphological correlates at photoreceptor ultrastructural levels are discussed. Optical coherency is implicated in these effects.