Bioactivity-guided fractionation of a methanolic CHCl₃ 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 (IC₅₀=9.3-20.0 μg/ml); [4] and [5] were moderately cytotoxic to CHO Aw B cells (IC₅₀=8.5-13.0 μg/ml).