Schedule-controlled behavior in infant and juvenile monkeys exposed to lead from birth.

Monkeys (Macaca fascicularis) were dosed orally from birth with 0 or 2000 micrograms/kg/day of lead as lead acetate. Blood lead concentrations of treated monkeys peaked at an average of 115 micrograms/dl by 100 days of age, and decreased to a steady state level of 33 micrograms/dl after withdrawal from infant formula at 270 days of age. No overt signs of toxicity were observed. Beginning at 60 days of age, monkeys were tested on a fixed ratio (FR) schedule of reinforcement, followed by a fixed interval (FI) schedule. Infants were tested in their home cages for 16 hours each day. When these monkeys reached three years of age, performance on a multiple fixed interval-fixed ratio schedule was evaluated. Infant performance was characterized by increased FR pause and decreased FI pause in the treated monkeys. Juvenile performance of lead-treated monkeys was characterized by increased Fl run rate, pause time, and index of curvature. Treated monkeys exhibited increased variability of performance both within and between sessions on several measures of Fl and FR performance.