Histochemical studies were made on the developing chick embryo heart to determine the effects of nicotine on acid mucopolysaccharide content during the critical stages of the cardiac morphogenesis. Two-day old embryos were injected with the dosages of nicotine ranging from 1.5 to 3 mg per embryo. The embryos were studied on the 3rd and 4th days of incubation. For an evaluation of acid mucopolysaccharides, the staining procedures of Mowry (1958) and Saunders (1964) were employed. It was found that nicotine decreases the content of acid mucopolysaccharide in the developing cardiac jelly. In the control embryos, the cardiac tissues picked up very intense coloration. At lower dosages (Group A; 1.5 mg/egg), the staining response of the treated heart tissues containing acid mucopolysaccharides was moderate as compared to the control embryos. At higher dosages (Group B; 3 mg/egg), the cardiac tissues were faintly colored as compared to the embryos of Group A. It is possible that the cardiac lesions previously reported in the chick embryos following the administration of nicotine (Gilani, 1971) are due to the reduction of the amount of acid mucopolysaccharides in the developing cardiac jelly--a susceptible period of the genesis of heart.