There is suggestive evidence that suicidal behaviour by itself may have biological correlates, regardless of which particular psychiatric illness it is associated with. Most of the available evidence concerns the neurotransmitter serotonin but there are also reports of deviations in other transmitters. This evidence is reviewed in this paper. The association of neuropeptides, cortisol, & cholesterol with suicide or suicidal behaviours is also discussed. (161 refs.)