1. The ionic current evoked by muscarinic receptor agonists was investigated in dispersed chromaffin cells of the guinea-pig adrenal medulla using the whole-cell version of the patch-clamp procedure. 2. Muscarine or oxotremorine (0.03-10 microns) produced an inward current associated with an increase in current noise at a holding potential of -40 mV. The relationship between current and oxotremorine concentration fitted well to a rectangular hyperbole with an apparent dissociation constant (KA) of 0.23 micron. 3. The muscarinic antagonists pirenzepine (0.1 micronM) and AF-DX 116 (0.3 microM) shifted the dose-response curve to the right in a parallel manner. Dissociation constants (KB) for pirenzepine and AF-DX 116 were estimated to be 50 and 70 nM, respectively. 4. The current-voltage relation for the current induced by muscarine had a negative slope below -30 or -20 mV, and the current reversed its polarity at 0.4 +/- 0.8 mV (n = 4) in standard salt solution. Removal of Mg2+ or Ca2+ from the perfusate did not modify the muscarinic current-voltage relationship. 5. When Na+ in the bath solution was replaced with Tris, the muscarinic current-voltage relationship shifted to the left (the hyperpolarizing direction); the current reversed its polarity at -18.7 +/- 1.2 mV in a solution containing 72 mM-Na+ (three cells) and at -57.5 +/- 2.7 mV in nominally Na(+)-free solution (three cells). When Ca2+ was replaced by Mg2+, in Na(+)-free solution, an inward current was not evoked by muscarinic stimulation. 6. Tetraethylammonium (TEA; 0.03-3 mM) reduced the muscarinic current at -40 mV, and the KD value was 0.34 mM with a Hill coefficient of 1. Barium (4 mM) reduced the current to 0.69 +/- 0.06 of control (n = 3). 7. When the recording electrodes contained guanosine-5'-O-(3-thiophosphate) (GTP gamma S, 100 microM), an inward current developed at -55 mV during the first few minutes after breaking into the cell interior. This inward current was associated with an increase in noise and was not larger at -70 mV than at -55 mV; it reached a peak value about 3-4 min after breaking into the cell interior and then declined over the next 2-3 min. 8. Muscarinic agonists had no effect in those cells in which intracellular GTP gamma S had first evoked a transient inward current. The inward current caused by nicotine was unaffected by intracellular GTP gamma S.(ABSTRACT TRUNCATED AT 400 WORDS)