To evaluate their toxicity in the developing brain, eight metal compounds, [bismuth sodium tartrate (BiNA-tartrate), CdCl(2), CoCl(2), HgCl(2), dimethyl mercury, NiCl(2), TlCl and triethyltin chloride (TET)] were tested in aggregating cell cultures of foetal rat telencephalon. The compounds were applied to the cultures continuously, either during an early developmental stage (between days 5 and 14) or during and advanced stage of maturation (between days 24 and 34). Changes in the activities of cell type-specific enzymes were used as a criterion for toxicity. A general cytotoxic effect was observed after treatment with either CdCl(2), HgCl(2) or TET at 10(-6)m, and with TlCl at 10(-5)m. Selective effects were found with BiNa-tartrate and dimethylmercury. CoCl(2) did not modify the parameters tested, whereas a stimulant effect was found with NiCl(2). The effects of several compounds were development dependent: HgCl(2), TET and TlCl were more toxic in immature cultures, whereas BiNa-tartrate, dimethylmercury and NiCl(2) were more effective in differentiated cultures.