The cadmilm-induced (Cd) damage of mammalian testes is thought to be correlated with an inhibition of carbonic anhydrase (CAH) by Cd. Since Cd causes dose-dependent changes in blood flow of the testes, an inhibition of CAH in the testes could be simulated by a decrease of CAH-rich erythrocytes. Therefore, CAH activities and hemoglobin (Hb) content were determined in blood and testes of untreated and Cd-treated Sprague-Dawley rats as well as in testes perfused via the testicular artery. Cd was intraperitoneally applied as CdCl2 in single doses of 1.5, 3.0, and 5.0 mg Cd2+/kg b.w., respectively. 1. The experiments on perfused testes clearly demonstrated that the CAH activities originate from erythrocytes rather than from a tissue located enzyme. 2. The alterations in blood circulation occurring shortly (0.25-1.0 h) after the Cd administration were characterized by a dose-dependent, transient decrease (1.5 mg Cd2+/kg) as well as an increase (3.0 and 5.0 mg Ck2+, respectively) of the Hb content in the testes. 3. Independent of these minor alterations in a later state (14-24 h after 1.5 mg Ck2+/kg, 7-14 h after 3.0 mg Cd2+/kg, and 1-3 after 5.0 mg Cd2+/kg), Cd induced the well known hemorrhagic alterations of the testes with a high increase of Hb content and CAH activity. 4. By means of the correlations between CAH activities and Hb content in blood and testes an inhibition of the CAH by Cd as the primary cause for the tissue damage of the testes could largely be excluded.