The goal of the program is to clarify the relations between events in ontogeny and the development, maintenance, and extinction of dyadic behaviors associated with aggression. Toward this end, the work will focus upon the mechanisms by which prior experiences serve to influence reactivity to others. The long-range objective of this work is to contribute to the establishment of empirical and theoretical generalizations on aggressive behavior development in mammals. Accordingly, detailed accounts of the processes of development in selected species are required to determine which mechanisms are relatively specific and which are general. The species selected for the present ontogenetic analysis are inbred mouse strains. Both observational and experimental procedures will be employed in the studies. A behavior coding scheme for the analysis of reciprocal social behavior patterns, developed in earlier work, can be extended to permit a detailed account of interactions over time. One of the principal experimental tasks will be to isolate and independently control those variables by which the effects have been thought to be mediated. A related problem to which the work will be addressed is the specification of conditions under which the effects of prior social interactions can be modified or reversed at maturity. To deal with these issues, a wide range of procedures will of necessity be employed, ranging from semi-naturalistic observations to the use of isolation rearing and maintenance in a device that mimics intraspecific social stimulation. In summary, the work will attempt to provide a coherent account of the semi-independent mechanisms through which experiential factors influence the individual and the interactions in which it becomes involved.