This study aimed to examine (a) whether findings of increased cardiovascular reactivity in relation to cognitive ability seen in infants, young adults and the elderly can be extended to middle childhood, and (b) which specific aspect(s) of intelligence is related to cardiovascular reactivity. We examined cardiovascular activity in 340 8- and 9-year-old children during a number judgment task and measured fluid and crystalized IQ using the WISC-IV (Wechsler, 2003). Regression analyses revealed that heart rate (HR) reactivity was positively associated with fluid intelligence and perceptual reasoning in particular, after controlling for the effects of sex, age, task performance, social adversity, and resting HR. Intelligence scores were not associated with respiratory sinus arrhythmia (RSA) reactivity. Findings are consistent with prior literature in infants and older populations and for the first time suggest that the association between HR reactivity and cognitive ability is specific for fluid reasoning.