Guatteria species (Annonaceae) are rich in isoquinoline alkaloids that contain a restricted β-phenethylamine moiety and thus have potential as monoamine oxidase (MAO) inhibitors for the treatment of Alzheimer's and Parkinsons diseases. Three aporphine alkaloids have been isolated from G. stenopetala, the known oxoaporphine dicentrinone (1), and two novel compounds, N-methyldicentrinone (2) and 4,5-dioxodehydrodicentrine (3). Their inhibitory effects on MAO-B were evaluated, but none of them was significantly active.