Learning and performance of two-way avoidance were investigated in a total of 68 rats trained with either a visual (change in illumination to darkness or to light) or an auditory (white noise of 70 or 60 dB intensity) conditioned stimulus (CS). Experiment I showed that the darkness CS produce lower avoidance performance and a much higher rate of intertrial responses than either a auditory or a compound (visual plus auditory) CS. A monotonic within-session increase of avoidance performance and a similar, but less regular increase of intertrial responses were found at the beginning of training in each group. In later sessions such trends were observed only in rats trained with a visual CS. Experiments II and III showed also rapid transfer of avoidance response and a corresponding change of intertrial response rate with the change of CS modality. When the compound CS was used, the effects of the visual element were completely overcome by the auditory one. Rats trained with a visual CS in Experiment IV showed a positive correlation between avoidance performance and the number of intertrial responses, which was more pronounced in earlier than in later training sessions. We consider the rise of intertrial behaviour as an adaptive response to the increase of task difficulty. CSi of different modalities differ not only in relative saliency, but also in the discriminability between their onset and offset. The modality of the CS influences not only avoidance performance but also the course of learning.