The aim of the research is to elucidate some of the processes that organize and coordinate the development of behavior in the rat, from birth to the time it leaves the nest. PART 1 is concerned with phenomena termed "group behavioral processes," which are dynamic behavioral and physiological attributes of the litter-aggregate. I review and extend my recent work on the development of behavior in the group, an approach that presents a different perspective of ontogenesis than that derived from studies of pups out of their typical context. This work includes studies which: (1.) Fractionate the melange of nest-typical stimuli into separate components in order to evaluate their contribution to the development of affiliative behaviors and social attractions among the juneviles. (2.) Examine how various group processes-function in active relationship with maternal behavior in a naturalistic nest environment. (3.) Evaluate the controls and consequences of these group and interactive behaviors, involving the mother-litter dyad, in terms of energy homeostasis. PART 2 of the proposal derives from recognition of the pre-eminent role of olfaction during early life and its crucial determinative function for future development. It is also recognized that the olfactory system is neurologically incomplete while it is performing vital functions for the pups. A set of experiments are designed to: (1.) Measure the development of chemosentivity in the rat pup, using novel methods for "developmental olfactometry." (2.) Examine the roles of experience in the development of olfaction and the function of the sense of smell in early life. (3.) Delineate how the quantity and quality of olfactory input influences the development of olfactory function, with correlative emphasis on the effects of olfactory input on neuroanatomical maturation in the olfactory system.