The effect of zinc and copper on the in vitro development of mouse preimplantation embryos and on metallothionein (MT) levels was studied by exposing the embryos to 100 μM concentrations of the metals for 24 h at the 1-cell, 2-cell, 4-cell, 6-8-cell, morula and blastocyst stages. Zinc affected embryo development in the early but not in the late stages, whereas copper affected it more generally. The combined presence of both metals caused a stronger embryotoxicity. MT levels were measured by radioimmunoassay and were found to be similar at all developmental stages, though possibly higher at the blastocyst stage. The exposure of embryos to zinc and copper increased MT levels significantly only at the blastocyst stage, supporting previously published results on MT mRNA levels. © 1993, Cambridge University Press. All rights reserved.