Preeclampsia is a multisystemic syndrome during pregnancy that is often associated with intrauterine growth retardation. Immunologic dysregulation, involving T cells, is implicated in the pathogenesis. The aim of this study was to evaluate the effect of upregulating regulatory T cells in an established transgenic rat model for preeclampsia. Application of superagonistic monoclonal antibody for CD28 has been shown to effectively upregulate regulatory T cells. In the first protocol (treatment protocol), we applied 1 mg of CD28 superagonist or control antibody on days 11 and 15 of pregnancy. In the second protocol (prevention protocol), the superagonist or control antibody was applied on days 1, 5, and 9. Superagonist increased regulatory T cells in circulation and placenta from 8.49±2.09% of CD4-positive T cells to 23.50±3.05% and from 3.85±1.45% to 23.27±7.64%, respectively. Blood pressure and albuminuria (30.6±15.1 versus 14.6±5.5 mg/d) were similar in the superagonist or control antibody–treated preeclamptic group for both protocols. Rats treated with CD28 superagonist showed increased pup weights in the prevention protocol (2.66±0.03 versus 2.37±0.05 g) and in the treatment protocol (3.04±0.04 versus 2.54±0.1 g). Intrauterine growth retardation, calculated by brain:liver weight ratio, was also decreased by the superagonist in both protocols. Further analysis of brain development revealed a 20% increase in brain volume by the superagonist. Induction of regulatory T cells in the circulation and the uteroplacental unit in an established preeclamptic rat model had no influence on maternal hypertension and proteinuria. However, it substantially improved fetal outcome by ameliorating intrauterine growth retardation.