Mothers and their firstborn year-old infants (30 girls, 30 boys) were observed during two counterbalanced situations. In the noninteraction situation, mothers were not to initiate interaction; during interaction mothers were free to interact. Maternal and infant behavior were dependent upon both the situation examined and how it compared to the previous situation. During interaction, mothers directed more behavior toward their infants than during noninteraction; this effect was intensified when noninteraction preceded interaction. Infant looking and proximity-seeking were greater during interaction than during noninteraction, while the reverse was true for touching and vocalization. A decrease in the level of interaction resulted in increases in infant behavior, while the reverse transition was associated with a more variable pattern. Two models describe the interrelationships between changes in maternal and infant behavior. In a reciprocal system, increases by the mother in a given behavior, or in the general interactive level are related to decreases in infant behavior, and vice versa. In an incremental system, any change in maternal behavior is met by increases in infant behavior.