ATP and several congeners were applied to locus coeruleus neurons in slices cut from rat pons. Whole-cell recording of membrane current showed that ATP caused an inward current at -60 mV. Effective concentrations (applied by superfusion) were 3-300 microM, and the peak current was about 150 pA at -60 mV. 2-Methylthioadenosine 5′- triphosphate was slightly more potent than ATP, adenosine 5′- diphosphate was about equipotent with ATP, alpha,beta-methylene adenosine 5′-triphosphate was slightly less potent than ATP, and beta,gamma'-methylene adenosine 5′-triphosphate and adenosine 5′- monophosphate had little or no effect. Adenosine (100 microM) caused small outward currents (40 pA). By changing the ionic composition of the pipette and extracellular solution, it was shown that the inward current resulted from both an increase in conductance to sodium ions and a reduction in conductance to potassium ions. It is concluded that rat locus coeruleus neurons express P2 purinoceptors, activation of which depolarizes the cells predominantly by increasing a conductance that allows sodium ions to enter the cell.