The contributions of genetic and environmental effects to reaction time (RT) variation in different experimental conditions were examined. A battery of 6 different RT tests measuring the speed of sensory-motor response was administered to 21 pairs of monozygotic (MZ) and 21 pairs of same-sex dizygotic (DZ) twins, aged 9-14. Auditory stimuli of 5 intensities were organized in sequences with constant inter-stimuli intervals. Tasks differed with respect to the stimuli sequence (regular or random), response (simple or choice RT) and response speed (fast response or unlimited response time). Behavioral genetic technique (model fitting) was used to estimate genetic and environmental effects in the observed RT measures. Four basic models were tested for each of the experimental conditions. The results of genetic analysis indicated that automated response in regular series was determined by genotype while more complex actions (choice RT, RT in random series) depended on environmental factors.