1. The cervical sympathetic trunk of the isolated rat superior cervical ganglion was stimulated with short bursts of repetitive pulses. At room temperature with rates of stimulation of 4 Hz and above, the ganglionic action potentials were reduced in size.2. Amphetamine (2.7 x 10(-5)M), which caused some depression of transmission during stimulation at 0.1 Hz, caused a partial reversal of the depression of transmission occurring with rates of stimulation above 4 Hz.3. This action of amphetamine was mimicked by adrenaline (3 x 10(-5)M) and noradrenaline (9.6 x 10(-5)M) but not by isoprenaline (1.8 x 10(-5)M) and was unaffected by propranolol (1.4 x 10(-5)M) but was abolished by prior application of phenoxybenzamine (5.8 x 10(-6)M). Furthermore, this action of amphetamine was unaltered in ganglia taken from rats pretreated with reserpine (single dose of 6 mg/kg, 16 h before dissection).4. Amphetamine had no effect on the surface potentials of the ganglion or on changes in these potentials produced by concentrations of carbachol (5. x 10(-6)M to 5.5 x 10(-3)M).5. It is concluded that amphetamine has a direct action on alpha-adrenoceptors situated at presynaptic sites.