In vivo assessment of toxicant action on Leydig cell function is subject to homeostatic mechanisms which make it difficult to determine whether any changes seen in serum testosterone (T) concentration are due to extragonadal endocrine alterations or to a direct effect on the Leydig cell. Studies used a testicular cell culture technique to evaluate Leydig cell testosterone biosynthesis in the presence of several metal cations. To determine the site of toxic action, the Leydig cells were stimulated to produce testosterone by using human chorionic gonadotrophin (hCG), dibutyl cyclic adenosine monophosphate (db cAMP) or several substrates required for the biosynthesis of testosterone. Ca(2+), Cr(3+), Fe(3+), Mg(2+), Na(1+) or Pb(2+) had no effect on stimulated testosterone. Dose response depression in both hCG and db-cAMP stimulated testosterone production were seen with Cd(2+), Co(2+), Cu(2+), Hg(2+), Ni(2+), and Zn(2+) treatment. Surprisingly several of the metal cations which caused a depression in hCG and db cAMP stimulated testosterone production caused significant increases in HCHOL and PREG stimulated testosterone production over untreated and similarly stimulated cultures.