Neurotransmitter receptors have been studied by conventional electrophysiological techniques in the membrane of human ovarian oocytes isolated from ovarian fragments obtained from pre-menopausal women undergoing abdominal surgery for gynaecological conditions. Ovarian oocytes respond to acetylcholine (ACh) concentrations as low as 10(-10) M by hyperpolarizing the membrane and by concomitantly increasing input resistance, in a dose-dependent manner. The response lasts as long as the transmitter is present in the extracellular fluid. No response is elicited by ionophoretically applied ACh. The ACh response has an apparent latency of less than 1 s and a reversal potential of about -12 mV. The response to ACh (10(-8) - 10(-3) M) is unaffected by curare (10(-5) - 10(-4) g/ml) and is blocked by atropine (10(-6) - 10(-4) g/ml). This indicates that ACh receptors in the human oocyte membrane are probably muscarinic in nature. No response is elicited by the amino acids glutamate, aspartate and glycine (up to 10(-3) M), or by noradrenaline, adrenaline and 5-hydroxytryptamine (up to 10(-3) M). On the basis of analogies to the response elicited by agents which activate parthenogenetic development in the oocytes of other mammals, it is suggested that the sperm-carried ACh might be involved in activation processes triggered by sperm-egg interaction.