Diamine oxidase (DAO; EC 1.4.3.6) levels are strongly reduced in epicotyls of 3-day-old etiolated lentil (Lens culinaris Medicus) seedlings upon exposure to continuous red and blue light, as compared to etiolated controls. Far-red light inhibits DAO activity to a lesser extent. A less marked effect can also be obtained by short (5-10 min) daily exposures. Phytochrome involvement in this light-mediated response has been demonstrated by red/far-red reversibility experiments. These findings provide the first evidence that mechanisms underlying the photoregulation of DAO level in the Leguminosae are related to photomorphogenesis and are essentially unrelated to the photosynthetic capacity of the seedling.