The bioassay-guided fractionation of the CH2Cl2 extract from the wood of Hyeronima oblonga (Euphorbiaceae) afforded two new alkaloids with a tetrahydroisoquinolinone nucleus described here for the first time. The compounds 3-methyl-4-methoxy-6-(1-oxoctyl.)-5,6,7,8-tetrahydro-1(2H) isoquinolinone (Hyeronine A, 4a) and 3-methyl-4-methoxy-6-(1-oxohexyl)-5,6, 7,8-tetrahydro-1(2H)isoquinolinone (Hyeronine B, 4b) displayed LC50 of 4 ppm in the brine shrimp lethality assay. (C) 1998 Elsevier Science Ltd. All rights reserved.