Abstract: We studied about population size, density, and structure, seasonal habitat use, and feeding habits of chital and blackbuck in the 2.7 km square Guindy National Park, Madras, Tamilnadu, using line transect sampling during 1991-92, and compared these with observations made during 1975-82 in the park. The chital population (density of 212.3/sq.km during 1991-92) has been stable or even increased between 1975 and 1992. Artificial feeding may be responsible for low infant and adult mortality and thus for maintaining very high chital density. On the other hand, the blacbuck population has declined sharply from about 250 animals in 1979 to about 85 (density of 17.5 sq.km) today. Demographic changes include a lower fertility and a lower recruitment into the young male age classes .The likely causes for the decline are habitat changes in blackbuck territorial areas and competition with chital. There was much overlap in space use and food items of the two ungulates. Management measures such as habitat manipulation, artificial feeding, control of exotics, and introduction of blackbuck from other areas may be needed for saving the blackbuck in this insular park, once considered a stronghold of this endangered species.