1. The ACh-activated K+ current (IK,ACh) has been investigated in guinea-pig atrial cells at 36 degrees C using the whole-cell patch-clamp technique. 2. During an exposure to ACh, IK,ACh faded as a result of desensitization. Throughout the fade of the current, the current reversed at EK and showed inward-going rectification. The fade was, therefore, the result of a genuine decrease in IK,ACh. 3. The onset of desensitization (as judged by the fade of IK,ACh) was biphasic and the time constants of the fast and slow phases of desensitization were 1.58 +/- 0.14 (n = 16) and 148.2 +/- 12.8 s (n = 18) respectively. Recovery from the fast and slow phases of desensitization (after 30 s and 5 min exposures to ACh respectively) occurred with time constants of 52 and 222 s respectively. This suggests that two processes are involved in desensitization. 4. The Q10 of the rate constant of the fast phase of desensitization was 2.2 +/- 0.3 (n = 6). 5. Intracellular perfusion with guanosine 5'-O-(3-thiotriphosphate) (GTP gamma S) or extracellular perfusion with AlF4- were used to bypass the muscarinic receptor and trigger IK,ACh by directly activating the G protein, GK, that links the muscarinic receptor to the K+ channel. Both GTP gamma S and AlF4- activated a current with the same reversal potential and the same degree of inward-going rectification as the ACh-activated current. 6. Desensitization still occurred when the muscarinic receptor was bypassed and IK,ACh was triggered by direct activation of GK with either GTP gamma S or AlF4-. This suggests that desensitization is, in part, the result of a modification of either GK or the K+ channel. 7. Activation of the muscarinic receptor by ACh resulted in greater desensitization than direct activation of GK; at the end of a 5 min exposure to ACh, current was only 22 +/- 1% (n = 19) of its peak value, whereas, after direct activation of GK by GTP gamma S for 5 min, current was 42 +/- 6% (n = 5) of its peak value. This suggests that desensitization also involves the muscarinic receptor. 8. When cells were perfused with GTP gamma S, the fast phase of desensitization could still occur, but the slow phase was reduced. This suggests that the fast phase involves GK or the K+ channel, whereas the slow phase involves the muscarinic receptor.(ABSTRACT TRUNCATED AT 400 WORDS)