Behavioral responses to injections of methylphenidate or saline in adolescent and early adult Spontaneously Hypertensive, Sprague Dawley, and Wistar Kyoto rats across the 10 pretreatment days. Filled symbols represent methylphenidate-treated animals. Open symbols represent saline-treated animals. Circles, squares and triangles represent Spontaneously Hypertensive, Sprague Dawley, and Wistar Kyoto rats, respectively. 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.