A leading role for prostatic levels of dihydrotestosterone (DHT) in the pathogenesis of benign prostatic hyperplasia is well established, if incompletely understood. The present study provides initial confirmation that 5 alpha-reductase inhibition alone is sufficient to prevent prostatic accumulation of DHT and to produce epithelial regression in the canine prostate. In dogs treated with the specific 5 alpha-reductase inhibitor finasteride, prostatic volume decreased to one-third of the baseline volume, while the prostatic concentration of DHT fell fivefold: both were constant in placebo control dogs. Demonstration that MR imaging can serve as accurate modality to assess prostatic volume was provided by serial measurements of the canine prostate and by correlation of the last imaging measurement with the weight of the excised prostate. Significant intensity changes were observed in T2-weighted images measured post-treatment; these changes correlated with the histopathology of the prostate. These results suggest that beyond quantifying regression, multiecho T2 measurements can be useful in probing accompanying changes occurring on the cellular level.