Microbial transformation studies of the cardiovascular drugs mexrenone (1) and canrenone (2) were conducted. Thirty-nine biotransformations of mexrenone and 84 biotransformations of canrenone were analyzed. Metabolism of the substrate was observed in the majority of these cases. Several monohydroxylated derivatives were detected by HPLC-MS-UV and subsequently identified. Two new mexrenone derivatives, 11alpha- (3) and 12beta-hydroxymexrenone (4), and the known metabolite 6beta-hydroxymexrenone (5) were isolated as major products produced by the Beauveria bassiana ATCC 13144 bioconversion (3) and the Mortierella isabellina bioconversion (4 and 5), respectively. Single-elimination products were also sought; however, only the production of the known metabolite Delta(1,2)-mexrenone (6) by several bacteria was confirmed. One new monohydroxylated derivative of canrenone, 9alpha-hydroxycanrenone (7), was isolated as a major product from the Corynespora cassiicola bioconversion. Structure elucidation of all metabolites was based on NMR and HRMS analyses.