Polyinosinic with polycytidylic acid (poly l with poly C), a double-stranded synthetic RNA, produced in newborn rats a runt syndrome characterized by mortality and retarded growth rates of the total body, thymus, and kidneys. In contrast, it induced a hyperplasia in the epidermis and in the spleen. Within 10 days of treatment, the epidermis became 2 or 3 times thicker and the spleen mass was increased by 50%. The epidermal hyperplasia involved all layers, but hair follicles were excluded. Splenic hyperplasia did not result from accelerated erythropoiesis. Double-stranded RNA was required; single-stranded homopolymers were ineffective. Theophylline, a phosphodiesterase inhibitor, did not potentiate the effects. The uptake of iododeoxyuridine-125 was not enhanced in the hyperplastic epidermis or spleen. Thus we concluded that poly l with poly C can retard the growth of some organs in newborn rats, but that it causes epidermis and spleen to accumulate cells. The cytokinetic mechanisms involved in these contrasting effects were not clear.