Serotonin, or 5-hydroxytryptamine (5-HT), is an important neurotransmitter^1,2 that activates the largest subtype family of G-protein-coupled receptors³. Drugs that target 5-HT_1A, 5-HT_1D, 5-HT_1E and other 5-HT receptors are used to treat numerous disorders⁴. 5-HT receptors have high levels of basal activity and are subject to regulation by lipids, but the structural basis for the lipid regulation and basal activation of these receptors and the pan-agonism of 5-HT remains unclear. Here we report five structures of 5-HT receptor–G-protein complexes: 5-HT_1A in the apo state, bound to 5-HT or bound to the antipsychotic drug aripiprazole; 5-HT_1D bound to 5-HT; and 5-HT_1E in complex with a 5-HT_1E- and 5-HT_1F-selective agonist, BRL-54443. Notably, the phospholipid phosphatidylinositol 4-phosphate is present at the G-protein–5-HT_1A interface, and is able to increase 5-HT_1A-mediated G-protein activity. The receptor transmembrane domain is surrounded by cholesterol molecules—particularly in the case of 5-HT_1A, in which cholesterol molecules are directly involved in shaping the ligand-binding pocket that determines the specificity for aripiprazol. Within the ligand-binding pocket of apo-5-HT_1A are structured water molecules that mimic 5-HT to activate the receptor. Together, our results address a long-standing question of how lipids and water molecules regulate G-protein-coupled receptors, reveal how 5-HT acts as a pan-agonist, and identify the determinants of drug recognition in 5-HT receptors.