Bioactivity-guided fractionation of a methanolic CHC13 extract of the leaves of Pterogyne nitens afforded the known guanidine alkaloid pterogynidine [2] and three new guanidine alkaloids, nitensidines A [3], B [4], and C [5], all of which exhibited selective activity towards the DNA repair-deficient yeast mutant RS 321 (IC12=93-20.0 μg/ml); 3,4, and 5 were moderately cytotoxic to CHO Aux B1 cells (IC50=8.5-13.0 μg/ml). © 1995, American Chemical Society. All rights reserved.