(-)-Propranolol displaced 5-HT1 and 5-HT1B receptor binding (pIC50 6.76 and 6.31 respectively) and antagonized the inhibitory effect of 5-HT on continuous K+ (25 mM) evoked release of [3H]5-HT from superfused rat frontal cortex slices (apparent pA2 6.67). (+)-Propranolol was essentially inactive in all tests. The results support the claim that the 5-HT autoreceptor has a pharmacological resemblance to the 5-HT1 recognition site and in particular to the low affinity 5-HT1B subtype of this site.