Behavioral responses to a challenge injection of cocaine in Spontaneously Hypertensive, Sprague Dawley, and Wistar Kyoto rats pretreated with methylphenidate or saline during adolescence or early adulthood. Only Spontaneously Hypertensive rats exhibit methylphenidate-induced behavioral cross- sensitization to cocaine. Adult SHR group pretreated with methylphendiate, n = 5. All other groups, n = 6. Please see text for statistical results. SHR: Spontaneously Hypertensive rat; SD: Sprague Dawley; WKY: Wistar Kyoto; MPH: methylphenidate; Sal: saline.