Three experiments are reported which concern the effects on the barpressing of fluid‐deprived rats of making the flavour of the fluid reinforcer conditionally aversive. In the first experiments, it was found that reinforced barpressing for a conditioned aversive flavour was less than that for a palatable flavour, but greater than nonreinforced barpressing. In the second experiment, it was found that conditioned aversive flavours supported less barpressing, and were consumed less, if they were presented in large quantities. In the third experiment, it was found that rats preferred to work for an unpaltable flavour delivered in small quantities than to drink it from a freely available spout. The quality‐quantity interaction is interpreted as evidence of a stimulus intensity dynamism eff