To understand transactional associations between mothers' biological stress responsivity and parenting behaviors, we examined bidirectional effects between maternal cortisol reactivity to observing their children during distress-eliciting paradigms and harsh parenting across infancy and toddlerhood using longitudinal data from the Family Life Project (N = 1,292, 41.5% African American). Children completed a series of distress-eliciting laboratory paradigms when they were 7, 15, and 24 months old, and mothers observed their children during the paradigms. Maternal cortisol reactivity was computed as a residualized change score from baseline to 20 min postparadigm, controlling for the time of day the saliva sample was collected. Harsh parenting was measured using five items from the Home Observation Measurement of the Environment inventory. A random-intercept cross-lagged panel model revealed that increases in maternal cortisol reactivity at 15 months predicted subsequent increases in harsh parenting at 24 months. Similarly, increases in harsh parenting at 15 months predicted increases in maternal cortisol reactivity at 24 months. Findings indicate that increased cortisol reactivity to children's distress in early toddlerhood may indicate a risk for harsh parenting in late toddlerhood and that increases in harsh parenting can also negatively impact mothers' stress physiology over time. (PsycInfo Database Record (c) 2024 APA, all rights reserved).