1. Fast desensitization of the muscarinic K+ channel has been studied in excised patches from rat atrial cells. 2. In inside-out patches, ACh was present in the pipette and GTP was applied via the bath to activate the channel. In outside-out patches, GTP was present in the pipette and ACh was applied via the bath to activate the channel. In both cases, during a 30 s exposure to GTP or ACh there was a decline in channel activity as a result of fast desensitization if ATP was present. 3. In inside-out patches, fast desensitization was still observed if the muscarinic ACh receptor was bypassed and the channel was activated by GTP gamma S. This suggests that fast desensitization is a result of a modification of the channel (or the connecting G protein) and not the receptor. 4. In both inside-out and outside-out patches, channel activity was depressed and fast desensitization was reduced or absent, if ATP was not present. 5. The non-hydrolysable analogue of ATP, AMP-PNP, did not substitute for ATP in its effects on the channel. 6. The results are consistent with the hypothesis that fast desensitization of the muscarinic K+ channel is the result of a dephosphorylation of the channel.