The aim of this work was to study the electrical and mechanical properties of small bundles of rat diaphragm muscle treated with two blockers of the delayed potassium rectification channels: 3,4-diaminopyridine (3,4-DAP, 2.5 mM) and tetraethylammonium (TEA, 20 mM). Twitch tension was significantly potentiated by TEA and 3,4-DAP (39% and 59% respectively). Maximal tetanic tension was not affected by both drugs. The voltage dependence of the tension vs the resting membrane potential was shifted to lower values in TEA and 3,4-DAP. 3,4-DAP increased the caffeine contracture tension (2.5–10 mM) and lowered the caffeine contracture threshold. The duration of the action potential (measured at the level of −40 mV) was increased by TEA and 3,4-DAP solutions. This change was a consequence of the decrease in the rat of repolarization of the action potential. In addition, TEA reduced the amplitude and the rate of rise of the action potential. We suggested that the increment in the duration of the action potential and the shift of the mechanical threshold to more negative values of membrane potential might be the factors involved in the twitch potentiation induced by the TEA and 3,4-DAP solutions.