Morphine, naloxone and pentobarbital were applied to ventral chemosensitive areas (M and S) and nonchemosensitive areas (pyramids) of the medulla oblongata in anesthetized cats. The respiratory and cardiovascular responses were studied. Morphine and pentobarbital caused a rapid respiratory depression when they were applied for 10 min to the areas M and S; they were inactive in the pyramidal area. The area M was more sensitive than area S to both drugs, because a more rapid and higher depression was induced in the area M. Morphine affected preferentially the respiratory frequency, whereas pentobarbital depressed tidal volume, regardless of the area tested. On the other hand, only pentobarbital reduced the blood pressure, the area S being more sensitive than the area M. When the contact of morphine was extended to 30 min, slight respiratory depression was also observed in the pyramidal area. Naloxone applied to the areas M and S reversed the effects of morphine only partially; i.v. injection of the antagonist was required to achieve full reversion and overshoot. The data show that the described areas are sensitive to opiate and nonopiate drugs. They seem to behave either as chemosensitive structures interacting differentially with drugs, or as areas that allow a rapid access of drugs into some centers deeply located in the brain stem.