EPSPs and spikes were recorded at rest and during rotation from single fibres of the posterior nerve in the isolated frog labyrinth. The spike discharge properties of 57 units were examined at rest and during repetitive acceleratory-velocity steps. Forty of these units were subjected to excitatory steps of 5-12 s duration and 45% displayed an evident discharge adaptation. In the non-adapting units, the excitatory response also deviated from that expected on the basis of the torsion-pendulum model and exhibited an exponential time-course in only 36% of the fibres examined. The time constant T2 of the response rising phase was significantly longer than that of the decay (2.5 s versus 1.7 s). When all the 57 units were considered, a linear behaviour was found in 67%. The average gain in these linear units was 1.9 +/- 1.4 spikes X s-1/deg X s-1. Adaptive fibres exhibited a lower resting firing rate and a higher gain (3.8 spikes/s and 2.3 spikes X s-1/deg X s-2, respectively) when compared with the non-adapting ones (7.1 spikes/s and 1.5 spikes X s-1/deg X s-2). An undershoot was present in 57% of the units; it increased with acceleration and was not strictly related to adaptation. Fifteen of the 40 units tested with the 5-12 s duration excitatory steps survived repeated inhibitory accelerations of the same duration. In these units a marked response asymmetry was evident since their resting activity could be abolished by accelerations not larger than 10 deg/s2.(ABSTRACT TRUNCATED AT 250 WORDS)