Citalopram together with fluoxetine, fluvoxamine, paroxetine and sertraline belong to the group of SSRIs, so named because of their pharmacological action as selective serotonin reuptake inhibitors in rat brain synaptosomes-their potency of inhibiting noradrenaline uptake is low and, from a clinical point of view, irrelevant. In contrast to classical tricyclic antidepressants and some antipsychotics, the SSRIs have little affinity for the dopamine D2 receptors, 5-HT1A and 5-HT2A receptors, alpha 1-receptors, beta-receptors, muscarinic receptors and histamine H1 receptors. Several authors have examined whether SSRIs are ligands of other receptors, including the 5-HT3-5-HT7 receptors and their subtypes, and the NMDA receptor, and whether chronic treatment with SSRIs modifies the affinity and binding capacity of these receptors. The SSRIs differ by their pharmacokinetics and pharmacogenetics of metabolism and by their cytochrome P450 isozyme inhibition properties. Some situations are presented in which plasma level monitoring of SSRIs is recommended, despite the lack of clearly defined "therapeutic windows'.