The occurrence of insulin receptors was investigated in freshly dissociated brain-cortical cells from mouse embryos. By analogy with classical insulin-binding cell types, binding of 125I-insulin to foetal brain-cortical cells was time- and pH-dependent, only partially reversible, and competed for by unlabelled insulin and closely related peptides. Desalanine-desasparagine-insulin, pig proinsulin, hagfish insulin and turkey insulin were respectively 2%, 4%, 2% and 200% as potent as bovine insulin in inhibiting 125I-insulin binding to brain-cortical cells, which corresponds to their relative biological potencies in classical insulin-target cells; no competition was observed with glucagon and nerve growth factor, even at high concentrations. Scatchard analysis of competitive-binding data resulted in curvilinear plots with a high-affinity binding of Ka = 3.6 X 10(8) M-1. Insulin binding to foetal brain-cortical cells differed, however, in two distinct aspects from that to classical insulin-binding cell types. Firstly, dilution of 125I-insulin-bound cells in the presence of unlabelled insulin did not accelerate dissociation of the labelled hormone. Secondly, exposure of brain-cortical cells to insulin before the binding assay enhanced insulin binding, suggesting up-regulation of insulin receptors in response to insulin. In conclusion, foetal-mouse brain-cortical cells bear specific binding sites for insulin. Their insulin receptor shows a marked specificity and affinity for insulin, but differs in at least two properties from most classical insulin receptors. These differences in hormone-receptor interaction could reflect structural differences between insulin receptors on embryonic and differentiated cells.