Methamphetamine (MA) induces neurotransmitter reductions and neurotoxicity at high doses in adult animals, but its effects on early brain development and behavior have received less attention. In this experiment the effects of MA exposure during a period equivalent to the human third trimester were examined. Rats (Sprague-Dawley CD) were injected subcutaneously with d-MA (30 mg/kg b.i.d.) early in postnatal development (days 1-10), later (postnatal days 11-20), or with water during both of these periods. Both early and later MA-exposed offspring exhibited augmented acoustic startle and impaired performance in a complex multiple-T water maze. Only the early MA exposure group showed a persistent deficit in weight while only the later MA exposure group showed impaired learning in the Morris hidden platform maze. Effects on locomoter activity are reported in the accompanying article. It was concluded that the effects of MA are both long lasting and stage dependent and involve cognitive as well as arousal functions.