The effects of hypertonic solutions on spontaneous transmitter release at synapses in the hypogastric ganglia of guinea-pigs were studied using intracellular recording techniques. 2. Hypertonic solutions were found to cause a marked hyperpolarization of the ganglion cells and this was accompanied by an increase in both the threshold and input resistance of these cells. 3. The frequency of spontaneous quantal release was found to depend exponentially on the tonicity of the bathing solution with a 50% increase in tonicity producing a 7-9 fold increase in the frequency of release. 4. Hypertonic solutions were also found to increase the degree of bursting within the spontaneous process so that the proportion of multiquantal spontaneous potentials was found to be increased in such solutions. 5. Two different models, the 'charge-screening' model (Van der Kloot & Kita, 1973) and the 'Ca-increase' model (Shimoni, Alnaes & Rahamimoff, 1977), for the mechanism underlying the effects of hypertonic solution were considered.
