Hypotheses were tested about how moment-by-moment variation in mothers' negative emotion predicts harsh and lax discipline. Mother-toddler dyads interacted in a task designed to elicit challenging child behavior. Mothers viewed videotapes of their interactions and continuously rated their experienced emotion. Harsh discipline was associated with mothers' greater negative emotion intensity, greater dependence of mothers' emotion on toddlers' negative affect, and lower serial predictability of mothers' emotion. Laxness was also associated with greater emotion dependence on negative toddler affect and lower serial predictability but not with negative emotion intensity. The dependence of mothers' emotion on toddlers' rule violations was not associated with discipline. Dynamic emotion variables were not redundant with emotion intensity and therefore enhance our understanding of the role of emotion in discipline.