Abstract. Effects of drugs acting at P2-purinoceptors on the release of newly taken up [3H]-noradrenaline were studied in slices of mouse and rat vas deferens. The slices were superfused and stimulated electrically, in most experiments by trains of 60 pulses/8Hz. In mouse vas deferens, the P2-purinoceptor antagonists reactive blue 2 (1.8100M) and brilliant blue G (10300M) increased the stimulation-evoked overflow of tritium in a concentration-dependent manner as shown previously for suramin. Reactive blue 2, which preferentially blocks the P2Y-subtype, was the most potent compound and the compound with highest maximal effect, an increase by 104%. Pyridoxalphosphate-6-azophenyl-2,4-disulfonic acid (PPADS), in contrast, caused a small increase only at a single concentration (30M). The effects of reactive blue 2, brilliant blue G and suramin were not additive. The P2 agonist adenosine 5-O-(3-thio)-triphosphate (ATPS) reduced the evoked overflow of tritium. As shown previously for suramin, reactive blue 2 30M and brilliant blue G 100M antagonized the effect of ATPS. From the shift of the ATPS concentration-response curve to the right, an apparent pKB value of 5.3 was estimated for reactive blue 2 and an apparent pKB of 4.5 for brilliant blue G. In rat vas deferens, reactive blue 2 (330M), brilliant blue G (10M) and suramin (30300M) also increased the evoked overflow of tritium. As in the mouse, reactive blue 2 was the most potent compound and the compound with highest maximal effect, an increase by 90%. As previously demonstrated in the mouse, suramin (300M) increased the evoked overflow of tritium only when rat vas deferens slices were stimulated by trains of 60 pulses at 1 or 8Hz, but not when they were stimulated by trains of 6 pulses/100Hz. The results confirm the operation of a P2-purinoceptor-mediated prejunctional negative feedback controlling the release of noradrenaline in mouse vas deferens and demonstrate the same mechanism in rat vas deferens. The prejunctional P2-purinoceptors are P2Y-like in both species. They are a novel kind of autoreceptors, operating in parallel to prejunctional 2-autoreceptors.