Staining for P-galactosidase (P-gal) reporter activity in adrenal glands from adult, fetal and neonatal 21-OH/LacZ transgenic mice revealed mosaic patch patterns that were qualitatively similar to those seen in LacZ wild-type mouse chimeras, at similar developmental stages. This suggests that, as in chimeras, the transgenic patch pattern may reflect cell lineage relationships. Consequently, 21-OHILacZ transgenic mice could be useful as a simpler alternative to chimeras for the analysis of clonal growth and cell mixing during adrenocortical organogenesis. Embryonic day 16.5 (E16.5) adrenal cortices of 21-OH/LacZ transgenic mice displayed a punctate patch pattern, but by E 18.5 "stripes" appeared to be emerging and were clearly visible by the day of birth (PO) and three days later (P3), consistent with the adult mosaic striped pattern. This suggests that adrenocortical organogenesis involves an initial period of randomly oriented clonal growth, followed by directional growth which begins in the perinatal period.