The impact of lead as an environmental pollutant on the I-131 uptake and retention in rat thyroid was assayed alone and in combination with lithium treatment. Lead treatment significantly stimulated the 2- and 24-h uptake of I-131 in the thyroid, and the 24-h uptake showed the maximum stimulation after 3 mo of lead treatment. On the contrary, lithium supplementation reduced the uptake significantly and the maximum decrease was noticed after 2 mo of lithium administration. Further, simultaneous lead and lithium treatment resulted in more pronounced increase in the uptake of I-131 by the thyroid, which was maximum after 3 mo of combined treatment. The thyroid biological half-life of I-131 (Tbiol) was found to be increased significantly following lead and lithium treatments when given separately. Interestingly, combined lead and lithium treatment given up to 2 mo further prolonged the Tbiol of I-131, thus reflecting its increased retention.