The X-ray crystal structures of three ergoline derivatives have been completed and are reported herein. These include mesulergine hydrochloride (III), pergolide methanesulfonate (IV), and dihydroergocriptine hydrate (V). These molecules show dopamine agonist activity at the D2 receptor. These three-dimensional structures were compared to structurally similar molecules with similar pharmacological activity. It was found that ergoline derivatives have rigid fused-ring systems with substituents that can exist in a variety of conformations. The ergoline structures reported here have structural features consistent with other active dopamine agonists. Crystallographic data: (III) C18H26N4O2S ⋅ HCl, monoclinic space group P21, a = 12.183(2) A, b = 6.174(2) A, c = 13.649(2) A, β = 105.55(1)°, Z = 2, final R = 0.071 for 2293 observed reflections [I > 3σ(I)]; (IV) C19H27N2S ⋅ CH3SO3, mono- clinic space group P21, a = 8.455(4) A, b = 29.868(4) A, c = 8.957(6) A, β = 111.95(5)°, Z = 4, final R = 0.055 for 3836 observed reflections; (V) C32H43N5O5 ⋅ H2O, monoclinic space group P21, a = 12.403(2) A, b = 18.103(3) A, c = 15.418(3) A, β = 113.04(2)°, Z = 4, final R = 0.058 for 3208 observed reflections.