1. Using perforated-patch, whole cell recording, we investigated the membrane mechanisms underlying O2 chemosensitivity in neonatal rat adrenomedullary chromaffin cells (AMC) bathed in extracellular solution containing tetrodotoxin (TTX; 0.5-1 microM), with or without blockers of calcium entry. 2. Under voltage clamp, low PO2 (0-15 mmHg) caused a graded and reversible suppression in macroscopic outward K+ current. The suppression during anoxia (PO2 = 0 mmHg) was approximately 35% (voltage step from -60 to +30 mV) and was due to a combination of several factors: (i) suppression of a cadmium-sensitive, Ca2+-dependent K+ current, IK(CaO2); (ii) suppression of a Ca2+-insensitive, delayed rectifier type K+ current, IK(VO2); (iii) activation of a glibenclamide- (and Ca2+)-sensitive current, IK(ATP). 3. During normoxia (PO2 = 150 mmHg), application of pinacidil (100 microM), an ATP-sensitive potassium channel (KATP) activator, increased outward current density by 45.0 +/- 7.0 pA pF-1 (step from -60 to + 30 mV), whereas the KATP blocker glibenclamide (50 microM) caused only a small suppression by 6.3 +/- 4.0 pA pF-1. In contrast, during anoxia the presence of glibenclamide resulted in a substantial reduction in outward current density by 24.9 +/- 7.9 pA pF-1, which far exceeded that seen in its absence. Thus, activation of IK(ATP) by anoxia appears to reduce the overall K+ current suppression attributable to the combined effects of IK(CaO2) and IK(VO2). 4. Pharmacological tests revealed that IK(CaO2) was carried predominantly by maxi-K+ or BK potassium channels, sensitive to 50-100 nM iberiotoxin; this current also accounted for the major portion (approximately 60%) of the anoxic suppression of outward current. Tetraethylammonium (TEA; 10-20 mM) blocked all of the anoxia-sensitive K+ currents recorded under voltage clamp, i.e. IK(CaO2), IK(VO2) and IK(ATP). 5. Under current clamp, anoxia depolarized neonatal AMC by 10-15 mV from a resting potential of approximately -55 mV. At least part of this depolarization persisted in the presence of either TEA, Cd2+, 4-aminopyridine or charybdotoxin, suggesting the presence of anoxia-sensitive mechanisms additionalto those revealed under voltage clamp. In Na+/Ca2+-free solutions, the membrane hyperpolarized, though at least a portion of the anoxia-induced depolarization persisted. 6. In the presence of glibenclamide, the anoxia-induced depolarization increased significantly to approximately 25 mV, suggesting that activation of KATP channels may function to attenuate the anoxia-induced depolarization or receptor potential.