Previous research has indicated that maternal depression and externalizing behavior can contribute to and exacerbate one other. The current study examined a bidirectional model and found that maternal depression and externalizing behavior interacted reciprocally, such that maternal depression at Grade 5 predicted externalizing behavior at Grade 6, and externalizing behavior at Grade 4 predicted maternal depression at Grade 6. Additionally, the current study examined inconsistent discipline as a mediator for the reciprocal relationships between maternal depression and externalizing behavior. Findings suggest that inconsistent discipline at Grade 5 mediated the relationship between Grade 4 maternal depression and Grade 6 externalizing behavior, but not the reverse relationship. Clinical implications are discussed.