This study investigated the reciprocity of prosocial behavior among 3- and 4-year-old Japanese preschool children during free-play time. Matrix correlation tests revealed positive correlations between the frequencies of object offering given and received within dyads and between the frequencies of helping given and received within dyads. These results suggest that young children reciprocate prosocial behavior spontaneously. Positive correlations were also found between the frequencies of object offering and helping behavior exchanged within dyads, suggesting that children exchanged the two types of prosocial behaviors (i.e., "interchanged"). The interchange was independent of both reciprocity within object offering and reciprocity within helping behavior in 4-year-olds. Friends reciprocated object offerings more frequently than non-friends, suggesting that friendship affects the quantitative aspect of reciprocity. These data provide refined evidence of reciprocity among children and also suggest that reciprocity becomes more complicated as children grow older. (Contains 5 tables.)