In vivo whole-cell patch-clamp recording provides a means for measuring membrane currents and potentials from individual cells in the intact animal. Patch-clamp methods have largely been developed in vitro. This body of work has contributed enormously to the understanding of many important phenomena in excitable cells--including synaptic plasticity in the mammalian central nervous system, and the behavior of individual protein channels. In recent years, an increasing number of groups have applied whole-cell recording techniques in the intact animal. Such in vivo studies offer the tantalizing possibility of uncovering the underlying principles and mechanisms of neural interactions within the natural context of fully intact biological networks. This unit focuses on strategies for overcoming the specific technical challenges posed by in vivo whole-cell recording. A straightforward procedure is described for obtaining whole-cell records from the cortex of the anesthetized rat; this procedure has also been applied successfully to awake animals and other rodent species with minor modifications.