The aim of this study was to characterize the type of 5 0 -deiodinase activity in the prostate of pubescent rats (7-8 weeks), to establish its distribution in the lobes (ventral, dorsolateral, and anterior), and to analyze its modulation by prolactin (PRL), testosterone, dihydrotestosterone (DHT), and 17b-estradiol (E 2 ). Our results showed that the enzymatic activity was highly susceptible to inhibition by 6-n-propyl-2-thiouracil and gold thioglucose, its preferential substrate was reverse tri-iodothyronine (rT 3 ), it exhibited a low dithiothreitol requirement (5 mM), and the apparent K m and V max values for substrate (rT 3 ) were approximately 0$25 mM and 9$0 pmol liberated/mg protein per hour, respectively. All these characteristics indicate the preferential expression of type 1 deiodinase (D1), which was corroborated by demonstrating the presence of D1 mRNA in prostate.D1 activity was detected in all lobes and was most abundant in the dorsolateral. Although we detected type 2 deiodinase (D2) mRNA expression, the D2 activity was almost undetectable. D1 activity was enhanced in animals with hyperthyroidism and hyperprolactinemia, in intact animals treated with finasteride (inhibitor of local DHT production), and in castrated animals with E 2 replacement. In contrast, activity diminished in castrated animals with testosterone replacement. Our results suggest that thyroid hormones, PRL, and E 2 exert a positive modulation on D1 activity, while testosterone and DHT exhibit an inhibitory effect. D1 activity may be associated with prostate maturation and/or function.