The toxic effects of cadmium (Cd) and mercury (Hg), as chloride salts, were studied using an hepatic human fetal cell line (WRL-68 cells). From viability curves and the proliferative capacity of the cell in the presence of the metal, three different cell treatments were chosen, (1) 0.5 microM of the metal chloride for 24 h (acute low dose treatment), (2) 0.5 microM of the metal chloride for 7 days (chronic treatment), and (3) 5 microM of the metal chloride for 24 h (acute high dose treatment). WRL-68 cells grown in the presence of Cd exhibited the same proliferative curve as control cells, whereas in the case of Hg, the cells increased their proliferative capacity. Both metals produced ultrastructural alterations in different degrees, mainly observed as mitochondrial and RER structural changes, depending of the treatment and concentration of the metal used. Cytotoxicity was assessed by measuring the release of lactate dehydrogenase from the cells. Acutely high dose-treated cells showed the highest value for this parameter, and Cd-treated cells presented higher lactate dehydrogenase release than the Hg-treated ones. Cell damage was also measured by alanine aminotransferase (ALAT) and aspartate aminotransferase (ASAT) activities. Acute high dose Cd treatment caused the highest value of enzymatic release. Lipid peroxidation was significantly different with respect to control cells in chronic and acute high dose treatments with both metals. Metallothionein (MT) induction in response to Hg treatment was not detected. However, a dramatic induction of this protein occurred in Cd-treated cells. WRL-68 cells differentially respond to Cd and Hg making this hepatic fetal human cell line a useful tool in investigating the mechanism of toxicity of these heavy metals.