The putative silent 5-HT1A receptor antagonist (S)-UH301 (S-5-fluoro-8-hydroxy-2-(di-n-propylamino)tetralin) dose-dependently enhanced (3 to 10 mg/kg, sc) plasma corticosterone levels in undisturbed rats, an effect shared with 5-HT1A receptor agonists. (S)-UH301 did not influence plasma glucose levels. This unexpected finding may be indicative of a partial 5-HT1A-receptor agonistic effect of (S)-UH301, or reveals a hitherto unknown mechanism of action in (S)-UH301.