Minces of testes from mature and immature guinea pigs were incubated with [14C]androstenedione. Other portions of the same testes were simultaneously incubated with [14C]testosterone. Under steady-state conditions, the predominant steroid was [14C]androstenedione in the presence of tissue from young animals and [14C]testosterone in that from mature animals, irrespective of the compound added initially. It is concluded that the steady-state equilibrium favours the weak androgen androstenedione in the presence of tissue from the young animals, but favours the potent testosterone in that from the mature animals. Portions of tissue were homogenized in phosphate buffer. The net reduction of [14C]androstenedione to [14C]testosterone was less by homogenates of testes from young animals than by those from mature animals. Doubling the concentration of the NADPH-generating system did not affect this difference between the homogenates.
