Morphological and anatomical effects of pyrazon, 5-amino-4-chloro-2-phenyl-3(2H)-pyridazinone, on Phaseolus vulgaris L. and Amaranthus retroflexus L. were studied after treatment of seeds or seedlings at various pyrazon concentration, exposure times and environmental regimes. Beans treated with pyrazon developed various chlorotic and necrotic symptoms on the unifoliate leaves. Chlorosis and necrosis were observed first on the leaf margins. Beans treated with pyrazon under a complete dark regime developed symptoms much later than those under an alternating light and dark regime. Pyrazon treated pigweed displayed symptoms similar to those on beans. Pyrazon induced abnormal chloroplasts and graded cellular collapse in the unifoliate leaves of beans and cotyledons of pigweed. The severity and/or extent to which morphological and anatomical symptoms developed were dependent upon herbicide concentration, exposure and treatment regime.