In the developing mouse, the Wilms' tumour gene, WT1, is first expressed in the intermediate mesenchyme lateral to the coelomic cavity (13 somite, early 9 dpc embryo). A few hours later, it is present around all the cavity and in the urogenital ridge (the earliest mesonephric tubules) and the differentiating heart mesothelium. By 11 dpc, expression is in the uninduced metanephric mesenchyme and in the presumptive motor neurons of the spinal cord. By 12.5 dpc, WT1 expression has increased in the induced mesenchyme of the kidney and a day later is particularly marked in the nephrogenic condensations. At 13.5 dpc, WT1 is briefly expressed in some differentiating body-wall musculature, while two days later, there is a small domain of expression in the roof of the fourth ventricle of the brain. By day 20, however, expression has become restricted to the kidney glomeruli. RNA-PCR analysis on 12.5 dpc embryos and on adult tissues shows that WT1 is weakly expressed in both eye and tongue. The expression pattern in human embryos (28-70 days) is very similar to that in the equivalent mouse stages (10-15 dpc). The results indicate that WT1 is mainly present in mesodermally derived tissues, although exceptions are ectodermally derived spinal cord and brain. The data indicate that WT1 plays a role in mediating some cases of the mesenchyme-to-epithelial transition, but its expression elsewhere argues that it has other tissue-specific roles in development.