1 Superfused guinea-pig cortical slices were electrically stimulated at different frequencies and the changes in acetylcholine (ACh) content measured. Synaptosomes were prepared at the end of the stimulation period and high affinity choline uptake (HACU) rate was measured. 2 The effect of increasing KC1 concentrations was compared on ACh content of the slices and on synaptosomal HACU. 3 Electrical stimulation (2, 5, 10, 20 Hz) elicited a frequency-dependent linear increase in synaptosomal HACU rate and a decrease in ACh content of the slices. 4 The addition of atropine (1.5 x 10(-8) M) to the slices enhanced and that of physostigmine (3 x 10(-5) M) reduced the frequency-dependent increase in HACU rate. Atropine (1.5 x 10(-6) M) not only antagonized the effect of physostigmine, but the HACU rate measured after treatment with both drugs was larger than that found after atropine alone. 5 These results indicate that in the cortical cholinergic nerve endings, depolarization caused by electrical stimulation is coupled with an increase in choline transport which can be modulated by the addition of atropine or physostigmine. Furthermore, within given experimental conditions a linear relationship exists between the reciprocal of ACh content in the slices and synaptosomal HACU.