An electrical resistance method was developed to measure volume changes in substratum-attached monolayer cultures. Astrocytes in primary monolayer cultures prepared from neonatal rat cerebral cortex were placed in a confined channel containing a balanced salt solution, and the electrical resistance of the channel was measured using an applied alternating current. If the volume of the cells increases, then the volume of the solution within the channel available for current flow decreases by the same amount, resulting in an increase in the measured resistance through the channel. If the volume of the cells decreases, a decrease in resistance would be recorded. This method allows continuous measurements of volume changes in real time. When primary astrocyte monolayers were exposed to hyposmotic solutions (93-193 mosmol/kgH2O), they showed a rapid initial swelling and, in the continued presence of hyposmotic media, a characteristic regulatory volume decrease (RVD) in which there was a return to normal cell volume within approximately 20 min. Astrocytes exposed to hyperosmotic media (343-493 mosmol/kgH2O) gave a decrease in electrical resistance, indicating shrinkage. Putative endogenous effectors of astrocytic swelling, such as high extracellular K+ and glutamate, resulted in a much slower onset of swelling and no sign of RVD. This system can reliably measure the average change in cell monolayer volume to 1-2% and thus provides a sensitive means of continuous measurements of changes in cell volume in monolayer cultures.