Normal pregnant guinea pigs and mice accumulate immune complex in their renal glomeruli. Evidence of immune complex included specific immunofluorescence finding of IgG deposition and typical electron dense deposits on the endothelial and mesangial sides of the glomerular basement membrane. In the guinea pig, C3 was found in the glomeruli; and half of the glomeruli exhibited histopathologic changes although obvious functional disturbance was not evidenced. In the mouse, IgG deposition was not accompanied by C3 or significant histologic changes; and the phenomenon was observed in seven of nine inbred strains of mice studied. Immune complex of pregnancy was found a) to increase with successive pregnancy; b) to appear in the second half of pregnancy; c) not to decrease following pregnancy; and d) to be present in pregnancy of syngeneic mating. Deposition of immune complex, on the other hand, was not found in pseudopregnant mice. The possible nature and significance of the immune complex found in normal pregnancy are discussed.