Attention deficit/hyperactivity disorder (ADHD), characterized by inappropriate levels of activity, attention, and impulsivity, has been suggested to be caused by changes in reinforcement and extinction processes possibly linked to dysfunctioning dopamine systems. The present study investigated reinforcement processes in spontaneously hypertensive rats (SHR), an animal model of ADHD. Using intra-cranial self-stimulation (ICSS), behavioral effects of varying current intensity, reinforcer density, and reinforcer delay were tested. Current was varied in order to find the weakest current producing the maximal response rate (optimal current) in the SHR and the controls during high (120 reinforcers/min) and low reinforcer densities (1 reinforcer/min). The results showed that optimal current was significantly lower in the SHR than in the controls during high reinforcer density while maximal response rates were not significantly different. During low reinforcer density, optimal current was not significantly different in the two strains, but maximal response rate was significantly higher in the SHR than in the controls. The SHR produced more responses during the testing of reinforcer density, but changes in reinforcer density affected response rates similarly in the two strains. The decrease in response rate as a function of reinforcer delay was more pronounced in the SHR than in the controls. Overall, more responses with short inter-response times (IRT) were found in the SHR compared to the controls during intermittent reinforcement. The results are consistent with a steepened delay-of-reinforcement gradient in SHR.