Electrical responses of the sensory epithelium of Lorenzinian ampullae as well as their nerve fibres frequency reactions to thermal stimulation were studied in Black Sea skates, Raja clavata. Electrically isolated ampullae with input resistance 500-800 K omega were used. They were loaded through a feedback circuit to a controllable resistance. Heating of the ampulla caused a lumen-negative shift of the transepithelial potential, its amplitude being approximately linearly dependent on the input resistance of the preparation (R1). Frequency responses of the nerve fibres were also dependent on R1. With R1 more (less) than 400-500 K omega there was an increase (decrease) in spike frequency during heating of the ampulla. Two temperature-dependent potential sources in sensory epithelium affecting spike frequency in the opposite ways are suggested.