The effect of uninephrectomy (UNX) or sham-operation (SO) in pregnant rats at the 5th-7th day of gestation upon pup kidney development was studied. In newborns, kidney wet weight was low in pups of the SO and UNX groups, kidney/body weight was low in UNX only; collagen was highest in UNX. At 6 weeks, wet weight was least in UNX; dry weight was low and %H2O high in SO and UNX; collagen/dry weight was high in SO and UNX. Maternal surgery (SO or UNX) appears to impose a non specific development deficit upon pups, although kidney collagen growth is sustained; no specific renotropic activity was evident in pups of the UNX groups.