While searching for novel nonsteroidal inhibitors of human and rat prostatic 5α-reductases, we found a new series of indoline and aniline derivatives that showed potent inhibitory activities for both enzymes. Among them, 3-chloro-4-{1-(4-phenoxybenzyl)indolin-5-yl]oxy}benzoic acid (2e, YM-36117) showed a more portent inhibitory activity for the human enzyme than ONO-3805 with an IC₅₀ value of 5.3 nM and a reduced rat prostatic dihydrotestosterone (DHT) concentration by oral administration. The synthesis and the structure-activity relationships of these indoline and aniline derivatives are presented.