Cytoplasmic lactate dehydrogenase (LDH), alpha-hydroxybutyrate dehydrogenase (HBDH), aspartate aminotransferase (AST) and alanine aminotransferase (ALT) enzymatic activities were measured in normal renal cortical tissue and in hypernephroma. Significantly lower activities were always found in tumoral tissue than in normal renal tissue. Their respective values (mean +/- SD) were: LDH, 4,333 +/- 747 (normal tissue) vs. 997 +/- 748 U/l (tumor); HBDH, 2,554 +/- 466 vs. 387 +/- 290 U/l; AST, 529 +/- 109 vs. 65 +/- 37 U/l, and ALT, 205 +/- 45 vs. 9.9 +/- 5.4 U/l. The LDH/HBDH ratio was significantly greater in tumoral (2.69 +/- 0.69) than in normal tissue (1.70 +/- 0.11). These results indicate that hypernephroma exhibits a low metabolic rate when compared to normal tissue. Their enzymatic activities suggest a decreased energy metabolism, predominantly of the anaerobic type, and a reduced synthesis of nonessential amino acids in the tumor. These findings could explain in part the slow growth rate of hypernephroma.