1. Voltage‐clamp Na+ currents (INa) were measured in rat fast‐twitch fibres using the loose‐patch‐clamp technique. Changes in the conditioning membrane potential produced slow changes in the peak INa elicited by short test depolarizations, due to a slow inactivation process. 2. Inactivation was increased by application of steady depolarizing potentials and was reversed by steady hyperpolarizations. These changes in peak INa could be well fitted by single‐exponential functions with time constants in the range of 1‐4 min. 3. The steady‐state values of the maximum peak INa at any potential could be well fitted by a function identical to the one describing the fast inactivation process. This gave a potential of ‐108 mV at which 50% of the channels were closed due to slow inactivation. 4. The maximum peak current densities obtained with the slow inactivation fully removed were as large as 20 mA cm‐2.