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NADPH-diaphorase histochemistry was used to identify and analyse the topography of cones in adult human retinae (in the age range 44 to 68 years). Retinae were treated for NADPH-diaphorase reactivity, present in vascular walls, amacrine cells and cone and rod outer segments; a subpopulation of less intensely reactive profiles, morphologically resembling cones, possibly represents the blue cone population. Regularly spaced pairs of cones (one intensely labelled and one weakly labelled cone) were also apparent throughout the retina, and were most common along the horizontal meridian, particularly towards the periphery. The diameters and distributions of labelled cone outer segments were assessed using image analysis. Cone density in the adult retina ranged from 2000 per mm2 in the temporal periphery, to 82,000 to 120,000 per mm2 at the fovea centralis. Distribution patterns confirmed the presence of a cone streak, extending from the foveal region into nasal retina, but no evidence of superior-inferior asymmetry was detected.