The present study tested eight boys with attention-deficit/hyperactivity disorder (ADHD) and 12 normal boys (comparison group), aged 7-12 years, to investigate the hypothesis that ADHD is associated with a steeper and shorter delay-of-reinforcement gradient than is normal. A two-component schedule of reinforcement was used to deliver trinkets or coins as reinforcers in a game-like test. One component was marked by a signal. During this period reinforcers (coins or trinkets) were delivered every 30 s. This component is called a 30-s fixed interval (FI) schedule of reinforcement and measures changes in reactivity to reinforcers. The other component was in effect when the signal was turned off. Then no reinforcer was ever delivered. This is called an extinction (EXT) component and measures primarily sustained attention. The ADHD children gradually developed hyperactivity to a large extent consisting of bursts of responses with short interresponse times (IRTs) during both schedule components. The response bursts not only constituted a substantial portion of the ADHD overactivity, but may well be a key component of the behaviour commonly described as impulsiveness, the key behavioural characteristic of ADHD. In addition, the ADHD children showed behaviour during the extinction component that may well be described as a sustained-attention deficit: initially stopping when the signal was turned off and then resuming responding some time thereafter as if the signal had been turned on again. The comparison group ceased responding during extinction and did not show impulsiveness. The findings were in accordance with a steeper and shorter delay gradient in ADHD.