The relationship of speed-of-information-processing (SIP), as derived from reaction times (RTs) on experimental tasks, and intelligence has been extensively studied. SIP is suggested to measure the efficiency with which subjects can perform basic cognitive operations underlying a wide range of intellectual abilities. Observed phenotypic correlations between RT and IQ typically are in the -0.2 to -0.4 range, and the question is addressed to what extent this relationship is determined by genetic or environmental influences. In a group of Dutch twins the heritabilities for RT tasks at age 16 and 18 years were estimated longitudinally and the nature of the RT-IQ relationship was investigated. At age 16 years heritabilities for a simple reaction time (SRT) and choice reaction time (CRT) were 64 and 62% and the average phenotypic correlations between the RTs and IQ, assessed by the Raven standard progressive matrices, was -0.21. At the second test occasion lower heritabilities were observed for the RTs, probably due to modifications in administration procedures. The mean correlations between the RTs and WAIS verbal and per formal subtests were -0.18 and -0.16. Multivariate genetic analyses at both ages showed that the RT-IQ correlations were explained by genetic influences. These results are in agreement with earlier findings (Baker et al., Behav Genet 1991;21:351-67; Ho et al., Behav Genet 1988;18:247-61) and support the existence of a common, heritable biological basis underlying the SIP-IQ relationship.