Evidence is presented to demonstrate that the rat is a sensitive rodent species for developmental immunotoxicity testing of chemicals. A battery of immune function assays was performed in adult rats, which were exposed perinatally (i.e., during gestational, lactational, and/or juvenile development) to three different classes of environmental chemicals. The chemicals employed were the following: the organotins di-n-octyltindichloride (DOTC) and tributyltin oxide (TBTO); the polyhalogenated aromatic hydrocarbon 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD); and the organochlorine pesticides methoxychlor (MXC) and heptachlor (HEP). Suppression of immune function was observed in adult rats exposed to each of these chemicals during immune system development. The duration of immune function suppression in the rats so exposed ranged from three weeks (i.e., DOTC and MXC) to 19 months (i.e., TCDD) after the last exposure to the chemical.