The latencies of go responses made during go, no-go discrimination training by monkeys were analysed. It was found that go responses to the positive stimulus (S +) were made on average more rapidly than go responses to the negative stimulus, even late in training. Further, a relationship between go latency and the a posteriori likelihood of S + having been presented was observed. This suggested that the precise duration of time during which a response is allowed may influence the level of performance in this task (perhaps also in operated monkeys) more strongly than has been supposed.