L-cysteine in doses of 1.2-1.3 mg/g, was administered to 4-day-old rats which were tested throughout development and as adults on a variety of behavioral tasks. Lesions of the type previously described from L-cysteine treatment were confirmed in several cortical and limbic structures. No neurodegenerative changes were observed in NaCl (10 mmoles/kg) controls. Surviving L-cysteine treated animals displayed no obvious impairments in motor ability or growth rate, but did show behavioral deficits when tested as adults on 3 behavioral tasks: spontaneous alternation; Lashley III maze; and pattern discrimination. Activity in the open field was significantly higher in the L-cysteine group at 20 days of age. Amphetamine, administered in doses of 1.5 to 3.0 mg/kg, had no differential effects on open-field activity in the two groups. The behavioral changes observed in L-cysteine treated animals is similar to that which has been reported in adult rats with extensive hippocampal damage.