Short exposures to low Pb concentrations (e.g., 25-50 ppm) have been shown to increase Fixed-Interval (FI) response rates while comparable exposures to higher concentrations (100-1000 ppm) typically decrease response rates. This study was based on the hypothesis that Pb body burden, in addition to Pb exposure concentration, is an important determinant of the nature of Pb-induced changes in Fixed-Interval (FI) response rates. One way to increase Pb body burden is to prolong exposure duration. Thus, by the above hypothesis, prolonging the exposure duration to low Pb concentrations could result in FI response rate decreases previously noted in response to higher concentrations of Pb. To test this contention, rats were exposed to 50 ppm Pb acetate in drinking water for 8 or 11 mon prior to assessment of performance on an FI 1 min schedule of food reinforcement. In contrast to the increased rates of responding previously reported after 1 mon of 50 ppm exposure, the 8-11 mon exposures produced decreased FI response rates relative to control values over 30-40 sessions. The decreased response rates were a function of longer interresponse times and consequently lower running rates; postreinforcement pause times and index of curvature were normal. Differential trends in response rates between control and 8 mon Pb-exposed rats reappeared when reinforcement schedule contingencies were changed from FI-1 min to FI-5 min and, again, when changed from FI-5 min to an FI-5 min clock schedule; no such differences were apparent when the original schedule parameter, FI-1 min, was then reimposed. Adult animals trained on FI schedules and then exposed to 50 or 500 ppm Pb for 3-5 mon showed no consistent subsequent performance changes. Taken together, these studies suggest a Pb-induced delay in the acquistion phase of FI-schedule controlled responding, while learned performance appears less vulnerable. However, a seemingly transient deficit may nonetheless reemerge when the environment imposes a new learning situation or an alteration in reinforcement contingencies. Such effects have important implications for human Pb exposure since behavioral transitions may be mandated throughout life.