Center for Research in Cognitive Neuroscience, National Center for Scientific Research, Marseille, France. ariehle@lnf.cnrs-mrs.fr Neuronal mechanisms underlying stimulus-response (S-R) associations in S-R compatibility tasks were identified in 2 experiments with monkeys. Visual stimuli were presented on the left and right calling for left-right movements under congruent and incongruent S-R mapping instructions. High- and low-pitched tones calling for left-right movements were presented to the left and right ear, and the stimulus side was irrelevant. Single neurons sensitive to the S-R mapping rule were found in the primary motor cortex. The large overlap between the neuronal populations sensitive to the stimulus side, the S-R mapping rule, and the response side, respectively, is consistent with the idea that sensory-to-motor transformation is a continuous rather than a discrete process. Results partly support the hypothesis that the increase in reaction time with incongruent mapping is caused by the automatic activation of the congruent, but erroneous, response.