Ropinirole hydrochloride, or diethyl[2-(2-oxo-2,3-dihydro-1H-indol-4-yl)ethyl]ammonium chloride, C16H25N2O+.Cl-, belongs to a class of new non-ergoline dopamine agonists which bind specifically to D2-like receptors with a selectivity similar to that of dopamine (D3 > D2 > D4). The N atom in the ethylamine side chain is protonated and there is a hydrogen bond between it and the Cl- ion. In the crystal structure, two cations and two anions form inversion-related cyclic dimers via N-H...Cl hydrogen bonds.