Piperoxan hydrochloride is an α₂ adrenoceptor antagonist. IC50 & Target: adrenoceptor^[1] In Vitro: When the medulla is superfused with α₂ adrenoceptor antagonist Piperoxane (50 μM; 5 min) while the pons is with artificial cerebrospinal fluid (ACSF), the three inactive preparations display rhythmic phrenic bursts at a low frequency (2-4 c/min), and the phrenic burst frequency of the 12 active ones significantly increases during the last 3 min of Piperoxane applications (163±12% of the previous mean frequency). In active medullary preparations, the effects of NA applications (25 μM; 5 min) are compared when the preparations sre superfused either by ACSF (n=8) or by the α2 adrenoceptor antagonist Piperoxane (50 μM; PIP-ACSF; n=5). NA applications either alone (NA-ACSF) or with Piperoxane (PIP-ACSF+NA) significantly increases the phrenic burst frequency. However, the blockage of the medullary α2 adrenoceptors by Piperoxane potentiates a phrenic burst frequency increase: during the fifth minute of NA applications, the phrenic burst frequency reached 171±11% of the mean control value when ACSF is applied alone and 234±21% of the mean control value when PIP-ACSF is applied in control condition^[1].