Neuronal activity has been shown to modulate the pH of the extracellular environment. Since neuronal circuits in the ventral horn of the spinal cord are highly active during patterned movements, and voltage-gated calcium channels play an important role in the production of spinal motoneuron output, the effects of changes in extracellular pH (pHe) on calcium currents in ventral horn neurons of the mouse spinal cord were examined. It is demonstrated that these channels are sensitive to modulation by pHe. The amplitude of the current mediated by these channels increased as the pHe was elevated. The elevated pHe also led to a hyperpolarizing shift in the voltage dependence of both activation and inactivation. The opposite effects were seen for a decrease in pHe. It was also noted that a decrease in pHe was associated with a faster inactivation of the current. It is concluded that voltage-gated calcium currents in ventral horn neurons are modulated by changes in pHe, and that this modulation may play a physiologically important role in determining motoneuronal excitability during behaviors such as locomotion.