Purino receptors of GFSHR-17 cells were characterized by perforated patch clamp and Ca2+ imaging techniques. Repetitive application of 5-25 mM ATP, for 5-10 s, every 3 min stimulated a depolarization of 5-10 mv followed by an increase in [Ca2+ ]i and a hyperpolarization. Long term application of ATP reduced the probability of repetition of stimulation. Similar results were obtained by application of UTP. The ATP (UTP) related electrical and Ca2+ signals were not affected by substitution of the extracellular Na+ by NMDG or choline or by presence of EGTA in bath solution. The hyperpolarization was suppressed by 35 mM K+ in the extracellular solution indicating an involvement of K+ channels for hyperpolarization. The ATP (UTP) dependent hyperpolarization as well as the Ca2+ signal were not affected by apamin or isoprotenol. However, inhibitors of PLC as well as IP3 receptor completely suppressed the ATP (UTP) dependent Ca2+ signal and hyperpolarization. Additionally blockade of the SERCA pump induced increase in [Ca2+ ]i and hyperpolarization even in absence of ATP (UTP). The results indicate that GFSHR-17 granulosa cells express P2Y receptors which activate K+ release via IP3 signal cascade. This signal cascade may be involved in regulation of the secretion of the antral follicular fluid during follicular maturation.