The effects of oral administration of methylphenidate (MPH) on delayed alternation performance in male rats. A. Representative dose/response curves from two individual rats. Results represent percent correct on the delayed alternation task following MPH administration. For most rats, a lower dose (1.0–2.0 mg/kg, p.o. 30 min) was found to improve performance, while higher doses often impaired performance (1.5–3.0 mg/kg). Rats showed individual differences in dose sensitivity. B. An optimal dose of MPH was found for all rats which significantly improved delayed alternation performance. Results represent mean ± S.E.M. percent correct on the delayed alternation task. VEH = cracker vehicle; MPH = optimal dose of methylphenidate (1.0–2.0 mg/kg); ** significantly different from VEH p = 0.002. C. Higher doses of MPH impaired performance and produced a perseverative pattern of errors. Perseveration was assessed by the greatest number of consecutive entries into a single arm of the T maze. Results represent mean ± S.E.M. number of consecutive entries. VEH = cracker vehicle; MPH = impairing dose of methylphenidate (1.5–3.0 mg/kg); * significantly different from VEH p = 0.046.