BACKGROUND Consequences of heroin abuse include organic damage of cerebral structures. The level of impairments is in a direct and positive relation with the length of heroin abuse. OBJECTIVE The aim of this research was the evaluation of the reaction time with heroin addicts with different length of substance abuse. METHODS METHODS 90 examinees were divided into three groups with relation to the length of heroin abuse. Data collection included a questionnaire referring to socio-demographic and addictive characteristics. A specially designed programme was used for the evaluation of reaction time to audio/ visual signal. RESULTS In relation to the reaction time as overall model, the difference between examinees with different length of heroin abuse can be found on the marginal level of significance (F = 1.69; df = 12; p = 0.07). In visual modality, with the increase of length of heroin abuse leads to a significant prolongation of simple (the first visual sign: F = 3.29; df = 2; p = 0.04) and choice reaction time (the second visual sign: F = 4.97; df = 2; p = 0.00; the third visual sign: F = 3.08; df = 2; p = 0.05). Longer heroin consumption also leads to the prolongation of the simple (the first auditory task: F = 3.41; df = 2; p = 0.04) and the complex auditory reaction time (the second auditory task: F = 5.67; df = 2; p = 0.01; the third auditory task: F = 6.42; df = 2; p = 0.00). CONCLUSIONS Heroin abuse leads to the prolongation of both simple and choice reaction time in visual as well as auditory modality. The average daily dose of opiates was the most important predictor of the abovementioned cognitive dysfunction.