Cultured neurons from the paravertebral sympathetic chain of rats possess excitatory P2X as well as excitatory uracil nucleotide-sensitive P2Y receptors. Preliminary observations had indicated that the analogous neurons of mice lacked P2X receptors. This difference was now investigated. Thoracolumbar sympathetic neurons from one- to three-day-old mice were cultured for seven days. When the neurons were preincubated with [3H]noradrenaline and then superfused, ATP failed to cause any change in tritium outflow. UTP (3-300 microM) and UDP (30-100 microM), in contrast, caused marked increases, and so did nicotine (3-100 microM). The effect of UTP was not changed by suramin but abolished by tetrodotoxin and in the absence of calcium. The effect of nicotine was antagonized by hexamethonium and also abolished by tetrodotoxin and in the absence of calcium. Pre-exposure to UDP prevented the effect of UTP. In neurons studied by means of whole-cell patch-clamp techniques under current clamp, ATP lacked any effect. UTP (100 microM), UDP (100 microM) and nicotine (10 microM) caused depolarization accompanied by action potentials. Pre-exposure to UDP prevented the effect of UTP. In neurons studied under voltage clamp, ATP, UTP and UDP failed to cause any detectable current. Nicotine (10 microM), in contrast, elicited inward currents. Neither UTP nor UDP reduced the M-type potassium outward current. These results demonstrate a pronounced difference between cultured sympathetic neurons from the mouse and the rat paravertebral chain. Neurons from both species possess the nicotinic acetylcholine receptor. Neurons from both species also possess uracil nucleotide-sensitive P2Y receptors which, when activated, mediate depolarization, action potential firing and noradrenaline release; these effects are not due to inhibition of M-type potassium channels. Only the rat but not the mouse neurons, however, possess P2X receptors which, when activated, mediate cation entry, depolarization, action potential generation and transmitter release. The absence of functional P2X receptors makes the mouse neurons suitable for further study of the uracil nucleotide-sensitive P2Y receptors.