"Proviron is an oral DHT steroid compound similar to Masteron. Although it is not an ideal compound for building muscle (actually it is not good at all for this purpose), Proviron is helpful in stacks because of its unique ability to keep the body from turning testosterone into estrogen, thus giving the testosterone a better anabolic effect.. . .Proviron is therefore best stacked with testosterone, which makes taking anti-estrogen compounds unnecessary."
"Competitive aromatase inhibitors, such as Cytadren, Arimidex, and probably Proviron, bind to the same binding site on the aromatase enzyme that testosterone does. By doing this, they allow less testosterone to bind to aromatase. So, less testosterone is converted to estradiol."
(FWIW -- Bill Roberts)
Please note I had links, but they violated T-Nation policy so they (correctly) removed them.
Clip and google to go find.