To evaluate the developmental toxicity of decabromodiphenylether (DBDE) after exposure during the period from late gestation to after lactation, maternal Sprague-Dawley rats were given DBDE at dietary concentrations of 0, 10, 100, and 1000 ppm from gestational day 10 (GD 10) to postnatal day (PND) 21. On PND 21 and 77, lymphocytes (Lymph) in the spleen, thymus, and peripheral blood of male pups were subjected to flow cytometric analyses for expression of surface markers [CD3, CD4, CD8a, CD25, CD45RA, CD71, and CD161(NKRP1A)]. On PND 21, the proportions of splenic CD4+ T cells in the 10-ppm group, activated B (CD45RA+CD71+) cells in the 100- to 1000-ppm groups, and activated T cells (CD3+CD71+) in the 1000-ppm group were significantly decreased, and the population of peripheral CD161+ natural kiler cells on PND 21 and 77 had decreased in the 100- to 1000-ppm groups. In the 1000-ppm group, the serum T3 level was significantly decreased on PND 21 and the serum T4 level was decreased on PND 77. The body, spleen, and thymus weights were not significantly decreased, but liver weight was significantly increased on PND 21 in the 10- to 1000-ppm groups. These results suggest that on PND 21, developmental exposure to the highest dose of DBDE had a weak immunomodulatory effect. Although the most of the immunomodulatory effect had recovered to normal levels on PND 77, a decreasing effect on the natural killer (NK) cell population remained.