This paper describes research which involved (1) the study of children's moral reasoning about their own spontaneous prosocial behavior and (2) an examination of the early development of such reasoning. Prosocial moral reasoning is defined as reasoning about conflicts in which the individual must choose between satisfying his or her own wants, needs and desires and those of others in contexts in which laws, punishment, authorities and formal obligations are irrelevant or deemphasized. Twenty-two nursery school children, from 48 to 63 months of age, were observed and questioned by a familiar experimenter, in the classroom setting, about their spontaneous helping, sharing and comforting behaviors over a 12-week period (approximately 4 hours per week). The children justified their prosocial behaviors primarily with references to others' needs and pragmatic considerations. In contrast to the findings of previous research, on prohibition-oriented reasoning, the children used little punishment and authority-oriented or hedonistic reasoning. No sex or age differences were found. (Author/BF)