The involvement of dopamine neurotransmission in behavioral responses to novelty is suggested by reports that reward is related to increased dopamine activity, that dopamine modulates exploratory behavior in animals, and that Parkinson's disease patients report diminished responses to novelty. Some studies have reported that polymorphisms of the human dopamine D4 receptor (D4R) gene are associated with personality inventory measures of the trait called "novelty-seeking". To explore a potential role for the D4R in behavioral responses to novelty, we evaluated D4R-knock-out (D4R-/-) and wild-type (D4R+/+) mice in three approach-avoidance paradigms: the open field, emergence, and novel object tests. These three paradigms differ in the degree to which they elicit approach, or exploratory behavior, and avoidance, or anxiety-related behavior. Thus, we used these three tests to determine whether the D4R primarily influences the exploratory or the anxious component of responses to approach-avoidance conflicts. D4R-/- mice were significantly less behaviorally responsive to novelty than D4R+/+ mice in all three tests. The largest phenotypic differences were observed in the novel object test, which maximizes approach behavior, and the smallest phenotypic differences were found in the open field test, which maximizes avoidance behavior. Hence, D4R-/- mice exhibit reductions in behavioral responses to novelty, reflecting a decrease in novelty-related exploration.