The serotonergic system in the brain modulates many types of behavioural and physiological processes. An example of this modulatory function is seen with the selective serotonin reuptake inhibitors (SSRIs) which enhance serotonin transmission and influence mood, anxiety states, aggression, feeding and sexual behaviour. At present, 14 different serotonin receptors have been described and, although the function and localization of many of these receptors is becoming increasingly clear, much remains unknown. The SSRIs are intriguing drugs; by blocking presynaptic and somatodendritic serotonin transporters, they enhance serotonergic neurotransmission and thereby activate serotonin receptors. It is this effect which leads to the characteristic effects of the SSRIs. Theoretically, however, it appears possible that they may have differential effects on the various subpopulations of serotonin receptors. Differences between the SSRIs have recently been reported in males with rapid ejaculation; fluvoxamine, in contrast to other SSRIs, did not affect rapid ejaculation. What difference in the mechanism of action between the SSRIs is responsible for this differential profile? A conditioned taste aversion procedure has been used in mice to investigate the discriminatory stimuli (cues) of fluvoxamine and fluoxetine. It appeared that the discriminatory stimulus of fluvoxamine is primarily mediated via 5-hydroxytryptamine (HT)1A receptors, whilst that of fluoxetine is primarily mediated via 5-HT2C receptors. Both types of receptors have been implicated in depression and it is conceivable that different SSRIs have intrinsic activity at these receptors. Investigations are now ongoing to determine whether this differential mechanism of action also applies to the other SSRIs and whether there are differences between the SSRIs with respect to their effect on sexual behaviour in rodents.