The voltage-clamp technique is applicable only to spherical cells. In non-spherical cells, such as neurons, the membrane potential is not clamped distal to the voltage-clamp electrode. This means that the current recorded by the voltage-clamp electrode is the sum of the local current and of axial currents from locations experiencing different membrane potentials. Furthermore, voltage-gated currents recorded from a non-spherical cell are, by definition, severely distorted due to the lack of space-clamp. Justifications for voltage-clamping in non-spherical cells are, firstly, that the lack of space-clamp is not severe in neurons with short dendrites. Secondly, passive cable theory may be invoked to justify application of voltage-clamp to branching neurons, suggesting that the potential decay is sufficiently shallow to allow spatial clamping of the neuron. Here, using numerical simulations, we show that the distortions of voltage-gated K⁺ and Ca²⁺ currents are substantial even in neurons with short dendrites. The simulations also demonstrate that passive cable theory cannot be used to justify voltage-clamping of neurons, due to significant shunting to the reversal potential of the voltage-gated conductance during channel activation. Some of the predictions made by the simulations were verified using somatic and dendritic voltage-clamp experiments in rat somatosensory cortex. Our results demonstrate that voltage-gated K⁺ and Ca²⁺ currents recorded from branching neurons are almost always severely distorted.