Activity of 29 histologically identified units in the SFO was recorded in parallel to blood pressure monitoring. The responsiveness of these units to angiotensin II (AII) applied either iontophoretically or via the carotid artery (IC) was investigated together with the capacity of the vasodilator substance NO2- to block the AII response. Fifty-five% of the SFO units responded to iontophoretic application of AII (vs. 27% in the surrounding areas). SFO units were also activated by blood pressure depressions, either as a mirror image (type B) or only restricted to the initial descendent phase of the hypotensive wave (type A). But 8 of the 29 units responded in a mixed A and B fashion, i.e., dramatic firing in the very beginning of the hypotension followed by a sustained moderate response until recovery. Hypertensive spontaneous fluctuations rarely and inconsistently elicited an inhibitory response. An unexpected phenomenon of a hypotensive response following IC injections of low doses of AII was observed and used in this investigation. These observations together with the finding that vasoplegics inhibited unit firing suggest that the SFO is equipped with units able to sense a reduction in local vessels' calibre and fire in relation to it. When activated chemically, electrically or mechanically these units may then trigger hypertensive, antidiuretic and drinking responses.