The expression of two zygotic genes (HE and BP10) during sea urchin embryogenesis was previously found to be early, transient, spatially restricted and controlled at the transcriptional level. Here we studied how the expression of these genes is affected when cell interactions are abolished by dissociating blastomeres and when development is perturbed by treatment with Li+. We found that in isolated blastomeres, transient transcriptional activity (HE) is unchanged and both genes apparently function in the appropriate cell type. Thus HE/BP10 expression is largely cell-autonomous and should rely on maternal factors unevenly distributed in the egg. Treatment with lithium does not affect the temporal control but decreases the transcriptional activity and the size of the domain of expression of the HE/BP10 genes. As the Li+ concentration increases, the border of the domain is progressively shifted towards the animal pole. This alteration of the spatial pattern is the earliest molecular evidence of a change in cell fate detectable only much later by morphological criteria, and reveals a gradient of sensitivity to Li+ along the animal--vegetal axis. These results suggest that the activity of the HE/BP10 genes is strongly dependent on spatially organized maternal information controlling early development.