As part of a longitudinal study of postpartum adjustment and infant development, this study examined whether securely attached 12-month-old infants show greater object mastery motivation and social-object mastery motivation than do insecurely attached infants. Object mastery relates to infants' goal-directed behavior that is oriented toward objects; social-object mastery relates to infants' efforts to involve other people in attempts to meet their goals in object play. The study also investigated whether certain environmental influences, such as maternal depression and maternal work outside the home, affect mastery motivation or attachment. Subjects were 95 middle class mothers with healthy first-born infants. Forty-seven mothers met criteria for major or minor depression. Within a month of the infant's 12-month birthday, subjects participated in the Strange Situation procedure and a mastery motivation experiment. All interactions were videotaped and coded for attachment and mastery behavior. Analysis revealed that social-object mastery was related to attachment security and infant gender. Securely attached girls exhibited higher social-object mastery scores than insecurely attached girls and securely and insecurely attached boys. No relationships between object mastery and maternal depression, maternal work outside the home, or infant gender were found. (Contains 27 references.) (MM)