Though there is no clear biochemical evidence suggesting any alteration in 5-HT metabolism or 5-HT receptor function in schizophrenia, many effective antipsychotic drugs, in addition to block ing DA-receptor also act as 5-HT2 receptor antagonists. 5-HT has a modulatory effect on DA pathways, so the two theories are not incompatible. Affinity for 5-HT2 receptors is a feature of many of the recently developed “atypical” antipsychotic drugs, which produce fewer extrapyramidal side-effects than the earlier DA-selective compounds. Consequently, it is postulated that a degree of 5-HT2 receptors block can counteract the undesirable side-effects associated with D2 receptor antagonists. It is also possible that 5-HT receptor antagonism contributes directly to the antipsychotic profile of these drugs, but opinions on this differ sharply.