1. Two noradrenergic components of contraction were induced by field nerve stimulation of the circular smooth muscle of guinea-pig vas deferens. One component occurred during the stimulation (former-response) and the other component occurred after stimulation had ceased (after-response), with a distinct tension fall between these two responses. 2. The effects of stimulation frequency and duration upon these components were examined. 3. Prazosin (0.05-0.5 microM) suppressed both responses, whereas nifedipine (0.02 microM) suppressed only the after-response. 4. It is suggested that in this tissue there are two neurogenic alpha-adrenergic contractions which possess different Ca2+ activation mechanisms. The former-response is due to an increase in cytoplasmic Ca2+ concentration through a pathway independent of action potentials, while the after-response is attributed to action potentials arising as a secondary response to the released noradrenaline.