Effective emotion regulation may promote resilience and preschool classroom adjustment by supporting adaptive peer interactions and engagement in learning activities. We investigated how hypothalamus-pituitary-adrenal axis (HPA) regulation, cardiac reactivity, and classroom emotion displays related to adjustment among low-income preschoolers attending Head Start. A total of 62 four-year-olds completed a laboratory session including a baseline soothing video; emotion-eliciting slides/video clips, and recovery. Salivary cortisol, heart rate, and vagal tone were measured throughout. Independent coders used handheld computers to observe classroom emotion expression/regulation. Teachers rated child motivation, persistence/attention, learning attitudes, and internalizing/externalizing symptoms. Results reveal associations between biobehavioral markers of regulatory capacity and early school adjustment.