A genetic analysis of simple reaction times (RT) measured in young twins is reported. Reaction times were obtained in a choice RT task with varying interstimulus intervals and in a mixed task condition that consisted of the same choice RT trials and of double task trials, where subjects simultaneously performed mental arithmetic and the choice RT task. All RT measures showed significant influence of common environment. Heritabilities were task dependent: for the choice RT task higher heritabilities were seen for shorter (2 and 3 sec) than for longer (4 and 5 sec) interstimulus intervals. In the mixed condition heritabilities of almost 50% were seen for RT measured in double task trials, whereas no genetic influence was observed for choice RT trials (3 and 5 sec interstimulus interval) in this condition. © 1991.