The effects of intracellular nucleotide triphosphates on time-dependent changes in muscarinic receptor cation currents (I_cat) were investigated using the whole cell patch-clamp technique in guinea pig ileal muscle. In the absence of nucleotide phosphates in the patch pipette, I_cat evoked every 10 min decayed progressively. This decay was slowed dose dependently by inclusion of millimolar concentrations of ATP in the pipette. This required a comparable concentration of Mg²⁺, was mimicked by UTP and CTP, and was attenuated by simultaneous application of alkaline phosphatase or inhibitors of tyrosine kinase. In contrast, a sudden photolytic release of millimolar ATP (probably in the free form) caused a marked suppression of I_cat. Submillimolar concentrations of GTP dose dependently increased the amplitude of I_cat as long as ATP and Mg²⁺ were in the pipette, but, in their absence, GTP was ineffective at preventing I_cat decay. The decay of I_cat was paralleled by altered voltage-dependent gating, i.e., a positive shift in the activation curve and reduction in the maximal conductance. It is thus likely that ATP exerts two reciprocal actions on I_cat, through Mg²⁺-dependent and -independent mechanisms, and that the enhancing effect of GTP on I_cat is essentially different from that of ATP.