Exposure of rhesus monkeys to lead during the first year after birth resulted in cognitive deficits when the monkeys were tested as adults (5-6 years of age). A pronounced lead-related deficit was detected in the test of Delayed Spatial Alternation (DSA), and a much less robust effect was detected in the Hamilton Search Task (HST). Both tests provided examples of "windows of sensitivity" to the effect of lead, where the behavioral criterion was challenging enough to elicit a deficit in lead-treated monkeys while still being within the capabilities of the controls. The lead-induced deficit in DSA was most pronounced after short intertrial delays, suggesting that the effect was probably not due to a mnemonic dysfunction, but rather may have been due to deficits in strategy or attention. The lose-shift type of error accounted for most of the lead-related DSA deficit, indicating that the lead-treated monkeys perseverated on an alternation strategy even when it was not rewarded. These results indicate that exposure to lead during the first year after birth can result in very long-term and possibly permanent cognitive deficits.