The cytosolic calcium concentration ([Ca2+]i) of the isolated outer hair cell of the guinea pig was measured using fluorescence imaging microscopy and the effects of efferent neuroregulators such as acetylcholine, ATP, GABA, substance P, enkephalin, calcitonin gene-related peptide, serotonin, dopamine, norepinephrine, and gluta-mate were investigated. Among the drugs tested only ATP induced an elevation of the [Ca2+]i of the outer hair cell. In the resting condition, [Ca2+]i averaged 104.5 ± 31.1 nM(n = 27), while 100 µM ATP significantly increased [Ca2+]i to 146.3 ± 43.5 nM (n = 19). Superfusion with Ca2+-free solution (pCa = 7.5) abolished the increase in [Ca2+]i induced by ATP, suggesting that ATP causes an entry of external Ca2+. The relevance of [Ca2+]i to the inhibitory actions of efferent neuroregulators is discuss