Presumably one function of an hypothesis is to provoke further thought and observation. Therefore, I welcome the critical comments of Ashcroft and colleagues (Journal, March 1981, 138, 268–9) on my suggestion concerning the relationship between dopaminergic disturbances and the positive and negative symptoms of schizophrenia (Crow, 1980a, b). However, I think they have misunderstood the proposal and have underestimated some of the relevant evidence. I believe the evidence for two processes is more compelling than they have conceded and I note that in their own hypothesis they have introduced two factors to account for phenomena that both Mackay (1980) and I regard as a problem for the simple dopamine theory.