In each of two studies, rats of different ages were given one prolonged open-field trial. Animals of 21 days and older displayed within-session activity decrements, while 15-day-olds failed to habituate. The abrupt transition from a pattern-of nonhabituation to one of habituation to a novel environment parallels the development of forebrain cholinergic mechanisms during the third and fourth postnatal weeks. These experiments conform to the suggestion that the 2-week-old rat is behaviorally similar to the hippocampectomized adult.In none of the studies of the ontogeny of rats' open-field behaviors originating in this laboratory (Bronstein, 1972) has there been any clear difference in habituation rates among the various age groups. The level of locomotor activity (e.g., square crossing) has always appeared as a decreasing function of apparatus exposure on any given day. The greatest activity decrement appears between the first and second minutes of a 5-min trial; however, such declining rates of ambulation have appeared rather uniformly among animals varying in age from 30 to 175 days.That younger rats habituate at a rate similar to that of the adults is an unexpected finding in the light of the many reports of very persistent play and exploratory behaviors among immature animals in a variety of settings (cf. Eibl-Eibesfeldt, 1970). Martinek and Lat (1968), for example, have shown that many behavior patterns of canines are slower to habituate in pups than in adults. And in an excellent review, Harper (1970) discusses some of the unique social and ecological pressures on young mammals that might have selected for persistently active and gregarious juveniles.One suggestion to account for the relatively undifferentiated rates of open-field habituation in the rat during the second through fourth months of life is that the youngest Ss may already be old enough to show the pattern of habituation typical of adults. Campbell, Lytle, and Fibiger (1969) and Douglas (1972) have speculated that the rapid development of the rat's hippocampus during the third and fourth postnatal weeks represents the onset of a mechanism, the integrity of which is essential for behavioral adjustments (habituation, extinction) to biologically insignificant stimuli (Carlton, 1963(Carlton, , 1969.The two studies presented here are tests of the hypothesis that 2-week-old rats display unusually persistent locomotor activity and are behaviorally much *This research was supported by Grant 1442 from the Research Foundation of the City University of New York and by Grant MH22027-o1 from the National Institute of Mental Health. We wish to thank Joseph Gavlick and David Small for their technical assistance during the preparation of this paper.tRequests for reprints should be addressed to Paul M. Bronstein, Department of Psychology, Brooklyn College, Brooklyn, New York 11210.like adults with bilateral hippocampal lesions in their failure to show an activity decrement with increased exposure to a new locale (Roberts, Dember, & Brodwick, 1962). Also, the t...