A number of dihydro-1,3-oxazinoquinolines and dihydro-1,3-pyridobenzoxazines have been synthesized by a Mannich-type reaction involving various hydroxyquinolines, paraformaldehyde, and amines. Some dihydro-1,3-oxazinoquinolinones were prepared by the cyclization of o-hydroxyquinolinecarboxylic acids with A]TV-dialkylcarbodiimides. All of the compounds were evaluated for antimalarial activity against Plasmodium berghei in mice and against Plasmodium gallinaceum in birds. 9-Amino-3-cyclohexyl-2-(cycIohexylimino)-2,3-dihydro-4/f-1,3-oxazino [5,6-c] quinolin-4-one and 2-benzyl-8-chloro-2,3-dihydro-10-methyl-l/f-pyrido[ 3,2-/]-1,3-benzoxazine produced statistically significant prolongation of life in mice infected with P. berghei. None of the remaining compounds exhibited appreciable anti-