The calcium sensitivity of a large conductance voltage-sensitive potassium channel found in cultured rat cerebellar Purkinje neurons was studied in membrane patches from the somatic region of the cultured Purkinje neurons using single-channel recording techniques. The potassium channel had a conductance of approximately 94 picosiemens (pS) under physiologic ionic conditions and was active at depolarized membrane potentials. Activity due to this channel type was not observed when the saline at the internal surface of the membrane was calcium free. Low intracellular calcium concentration (10 nM) triggered channel activity at depolarized membrane potentials. Channel activity increased further with increasing intracellular calcium concentrations and was evident at more negative membrane potentials. The high sensitivity of this potassium channel type to intracellular calcium and its abundance in the Purkinje neuron membrane may reflect a prominent role in the control of action potential duration and interspike interval when the neurons are firing in a rapid, repetitive mode.