Background: Several studies on serotonin 1A (5-HT1A) receptor knockout mice in different genetic backgrounds indicate that such mice display a more anxious phenotype than their corresponding wild types. We hypothesized that the 5-HT1A receptor knockout mice would show a different phenotype than the wild type mice in the stress-induced hyperthermia (SIH) paradigm, which tests putative anxiolytic effects of drugs. Moreover, on pharmacologic challenges with the 5-HT1A receptor agonist flesinoxan we expected an absence of the functional response in knockout mice relative to wild type mice. Methods: Effects of the 5-HT1A receptor agonist flesinoxan, alone or in combination with the 5-HT1A receptor antagonist WAY-100635, and the γ-aminobutyric acid A (GABAA)-benzodiazepine receptor agonist diazepam were studied in the SIH paradigm in male 129/Sv 5-HT1A receptor knockout and wild type mice. In addition, the effects of flesinoxan on plasma corticosterone concentrations were determined. Results: Plasma corticosterone concentrations were dose dependently elevated by flesinoxan in wild type mice but not in knockout mice. Flesinoxan dose dependently decreased SIH in wild type mice but not in knockout mice. The flesinoxan effect in wild type mice was blocked by WAY-100635. Furthermore, diazepam decreased SIH in both genotypes. There were no differences in basic SIH responses between wild type and knockout mice. Conclusions: 5 -HT1A receptor knockout mice display a normal SIH response, and results indicate, based on the SIH, that the GABAA-benzodiazepine receptor complex functions normally.