Pharmacological experiments have implicated a role for serotonin (5-HT)(1A) receptors in the modulation of anxiety. More recent is the interest in corticotropin-releasing hormone (CRH) system as a potential target for the treatment of anxiety disorders. However, selective pharmacological tools for the CRH system are limited, hampering research in this field. Gene targeting is a relatively new approach to study mechanisms underlying anxiety disorders. 5-HT(1A) receptor knockout (1AKO) mice have been created on three different background strains, and two different lines of mice, overexpressing CRH (CRH-OE), have been generated. In the present review, behavioural and physiological findings reported for 1AKO mice and CRH-OE mice will be reviewed. As behavioural phenotyping is often limited to one or two approach avoidance paradigms, we extended these observations and also tested 1AKO and CRH-OE mice in a conditioned fear paradigm. This paradigm reflects essentially different aspect of anxiety than approach avoidance paradigms. 1AKO mice on a 129/Sv background strain showed similar freezing as wild-type (WT) mice. In CRH-OE mice, less freezing was observed than in the corresponding wild-type mice. The fact that the anxious phenotype of these genetically altered mice seems less clear than initially reported will be discussed. Rather than studying the direct consequences of alterations in the targeted gene, 1AKO and CRH-OE mice seem very valuable to study compensatory processes that seem to have taken place in reaction to life-long changes in gene expression.