1. The release-inhibiting alpha2-adrenoceptors of cerebral serotoninergic axons were studied in mice. Slices of the hippocampus or the occipito-parietal cortex from NMRI mice, from mice lacking the alpha2A/D-, the alpha2B-, the alpha2C- or both the alpha2A/D- and the alpha2C-adrenoceptor, and from mice sharing the genetic background of the receptor-deficient animals (WT) were preincubated with [3H]-serotonin and then superfused and stimulated electrically, in most experiments by trains of 8 pulses at 100 Hz. 2. The concentration-response curves of the alpha2-adrenoceptor agonist medetomidine were virtually identical in hippocampal slices from NMRI and WT mice, with maximally 70% inhibition and an EC50 of about 2 nM. In hippocampal slices from NMRI mice, phentolamine and rauwolscine were equipotent antagonists against medetomidine. 3. The effect of medetomidine was greatly reduced, with maximally 20% inhibition, in hippocampal slices from alpha2A/D-adrenoceptor-deficient mice; was slightly reduced, with maximally 59% inhibition, in hippocampal slices from alpha2C-adrenoceptor-deficient mice; was not changed in hippocampal slices from alpha2B-adrenoceptor-deficient mice; and was abolished in hippocampal slices from mice lacking both the alpha2A/D- and the alpha2C-adrenoceptor. 3. Similar results were obtained in: (i) occipito-parietal slices from NMRI and alpha2A/D-adrenoceptor-deficient mice and (ii) hippocampal slices that were preincubated with [3H]-serotonin in the presence of oxaprotiline to rule out cross-labelling of noradrenergic axons. 5. The serotoninergic axons of the mouse brain possess both alpha2A/D-heteroreceptors, which predominate, and alpha2C-heteroreceptors but lack alpha2B-adrenoceptors. The situation resembles the coexistence of alpha2A/D- and alpha2C-autoreceptors but lack of alpha2B-autoreceptors at the noradrenergic axons of mice.