1. The outward membrane current underlying delayed rectification in uterine smooth muscle has been studied by means of a double sucrose gap apparatus with particular reference to the effects of the external calcium. 2. The outward current was reversibly reduced in calcium-free solution and in the presence of manganese (5 mM), or increased in high-calcium solution. 3. In reference solution, when depolarizing steps activated the outward current to its maximal value, the current tails measured at the end of the pulse were made up of two exponentially declining components. The slower of the two components was suppressed in calcium-free solution. The fast component reached full, steady-state activation at about +75 mV and the slow one at less positive potentials, i.e. +50 mV. Altering the external calcium did not shift the activation curves of the outward current along the voltage axis. 4. The reversal potential of the outward current was not affected by alterations of the external calcium concentration. 5. The outward current components can also be separated on the basis of their sensitivity to 4-aminopyridine (4-AP) and tetracethylammonium (TEA). The fast component was selectively blocked by externally applied 4-AP. TEA blocked both fast and slow components. 6. It is suggested that two sets of potassium channels contribute to the outward current in myometrium and that these channels can be separated pharmacologically.