This study investigated continuity and stability of hypothalamic-pituitary-adrenal (HPA) and sympathetic nervous system (SNS) response measures in mother-infant dyads across 2 different types of social stress sessions. Synchrony of response trajectories across systems (SNS-HPA coordination) and partners (mother-infant attunement) was addressed, as were associations with infant temperament. Primiparous mothers and their 18-month-old infants (n = 86 dyads) completed an attachment stressor--Strange Situation (Ainsworth, Blehar, Water & Wall, 1978)--at Session 1 and challenge stressors--cleanup task and emotion task battery--at Session 2. Mother and infant saliva samples collected to index pre-stress, stress, and post-stress response during each session were assayed for cortisol (HPA marker) and salivary alpha-amylase (sAA; SNS marker). Multilevel modeling of cortisol/sAA trajectories across sessions revealed rank-order stability in mother/infant stress measures but discontinuity in absolute levels; cortisol trajectories were higher during attachment stress, and sAA trajectories were higher during challenge stress. Varying degrees of mother-infant attunement were found across sessions/systems. Infant surgency predicted higher stress measures, and negative affect and effortful control predicted lower stress measures, though associations depended on session/system. Findings are discussed in terms of advancing a multisystemic, contextual definition of developing stress responsiveness. (Contains 3 figures, 3 tables, and 3 footnotes.)