Single-unit activity of prefrontal (PF), premotor (PM), primary motor (MI) and posterior parietal (PP) cortical areas was analysed from 3 monkeys trained to perform visually guided arm movements in a between-arms choice reaction time (RT) task. Percentages of preparation-related units, i.e. units whose change in activity during the preparatory period was correlated with RT, were 19% in PF, 31% in MI, 46% in PM and 68% in PP, respectively. These data support the hypothesis that neural pathways connecting PP association areas to MI, via PM, are involved mainly in movement planning.