The configuration and width of on- and off-response zones in the discharge field of single cells in cat striate cortex was analysed by quantitative methods. The responses across on- and off-zones were plotted for 321 cells with a stationary optimum oriented light slit. The cells fell into two completely distinct subgroups with respect to the degree of overlap between adjacent on- and off-zones. The simple cells had a mean overlap of 16.8%, the complex cells 94.5%. For simple cells the ratio between the maximum off- and maximum on-response in the discharge field was bimodal, showing that two distinct subgroups termed on- and off-dominant cells could be distinguished. For the complex cells the corresponding frequency distribution was unimodal. The maximum response on the two regions adjacent to the most responsive discharge zone (the dominant zone) differed markedly for most simple cells, and only a very few cells had discharge fields approximating an ideal even symmetric field. The frequency distribution of the ratio between the maximum response in the two regions was unimodal showing that odd and even symmetric fields did not form distinct subgroups of simple cells. The number of different discharge zones in simple cells varied from one to five. The zones were arranged as alternating on- and off-zones across the discharge field. The maximum response in the subzones decreased with increasing sequential distance from the dominant zone, so the response pattern across each side of the discharge field resembled a damped wave-form pattern. All the complex cells had one on- and off-zone which overlapped. The mean width of the subregions in the simple cell discharge field and the mean distance between the response maxima in the subzones increased in the same proportion with increasing eccentricity. The paracentral fields were therefore like magnified central fields. The average width of the whole discharge field was not significantly different for the simple and the complex cells at the various eccentricities.