The ultrastructure of the labyrinth of Lampetra fluviatilis has recently been described by Lowenstein, Osborne & Thornhill (1968). This is now followed by an account of electrophysiological experiments in which it is shown that the elaborate shape of the cristae in the two vertical canal ampullae and the topographic arrangement of the hair cells in the sensory epithelia of the cristae makes the two semicircular canals capable of responding to angular accelerations in all planes including the horizontal, despite the absence of the horizontal (external) canal. Sequences of integrated multifibre responses from the otolith organs are presented. They show that the static responses to constant-speed tilting and to lasting deviations of the head from the normal differ in no way from those found in the gnathostome labyrinth. Frequency-synchronized impulse responses to vibration of the substrate show that the vibration sensitivity of the lamprey labyrinth resembles that found in the elasmobranchs both with respect to threshold and frequency range. It is suggested that vibration reception is localized in the sacculus and macula neglecta. This view is supported by the topographic arrangement of hair cells in the sacculus macula which is identical with that found on the vibration-sensitive sacculus macula of elasmobranchs. Rhythmic bursts of discharge activity form part of the electric activity found in the eighth nerve of the lamprey. Its origin could not be located. The functional significance of the large ciliated chambers found in the lamprey labyrinth remains obscure.