Abstract The topographical distribution of reward and aversion effects to intracranial stimulation (ICS) through monopolar electrodes was investigated at 61 sites in the rat mesencephalon. In order to minimize interpretive difficulties all rats were tested with identical ICS frequency (50 Hz), pulse width (200 μsec) and current strength (150 μA). To increase the probability of reward and aversion effects being detected, the rats were required to perform a simple locomotor response in a shuttle-box. The sensitivity of the shuttle-box procedure in detecting reward and aversion effects is indicated by the fact that, in spite of the low ICS charge employed here, only 8% of the rats would neither initiate nor escape ICS. Reward effects were found with a low probability (30%) and were almost exclusively associated with stimulation of the catecholamine cell groups A8, A9 and A10. Aversion effects were found with a high probability (92%) and were uniformly distributed, being obtained in every anatomical structure investigated. It is suggested that mesencephalic ICS is generally characterized by negative effect and that involvement of the monoaminergic systems results in the superimposition of positive effect on this negative background.