Posterior urethral valves cause bladder outflow obstruction and damage to the developing fetal kidney. Posterior urethral valves affect 1 in 8000 new-born males. A third of these children develop end stage renal failure by adolescence, despite valve ablation in the early post-natal period, implying that majority of the damage to the kidneys occurs in utero. How does this damage occur, and should we intervene in utero? The answers to these questions require further research, and are the basis to this thesis. This thesis focused on the effect bladder outflow obstruction has on morphology and apoptosis in the fetal kidney in a fetal lamb model. It also looked at the effect of bladder outflow obstruction on fetal stress hormone levels. Bladder outflow obstruction was created surgically in fetal lambs at day 70 of gestation, and fetal kidneys were analysed at day 2, 5, 10, 20 and 30 after creation of obstruction. Controls undergoing sham surgery were used for comparison. Four aspects were investigated: - effects of bladder outflow obstruction on renal histology effects of bladder outflow obstruction on expression of pro-apoptosis gene Bax and anti-apoptosis gene Bcl-X - effects of bladder outflow obstruction on renal regional apoptosis effects of bladder outflow obstruction on serum fetal ACTH and cortisol levels. Bladder outflow obstruction resulted in sequential morphological change in the fetal kidney over time. By 2 days post-obstruction, cystic change was noted. In addition, patchy attenuation of the nephrogenic blastema was evident by 5 days post-obstruction, with more confluent blastemal attenuation as well as generalized renal architectural disorganization by 10 days post-obstruction. By 20 and 30 days post-obstruction, cystic renal dysplasia had developed. Bladder outflow obstruction resulted in an increase in the ratio of renal expression of pro-apoptosis gene Bax to anti-apoptosis gene Bcl-X. Regional apoptosis counts showed increased tubular apoptosis compared to controls at 2 days post-obstruction, and increased blastemal apoptosis compared to controls at 5 days post-obstruction. By 10 days post-obstruction, blastemal apoptosis counts were reduced compared to controls. There were no significant differences in fetal serum ACTH and cortisol levels between fetal lambs with bladder outflow obstruction and controls. In conclusion, the results of this thesis outline the spectrum of morphological change in the fetal kidney over 30 days of bladder outflow obstruction. They show that detectable changes in morphology occur within two days of bladder outflow obstruction. Likewise, detectable changes in gene expression occur within 2 days of bladder outflow obstruction. The increased ratio of expression of Bax to Bcl-X suggests a swing towards increased apoptosis in response to bladder outflow obstruction. Further research is required to ascertain if these changes are reversible. However, the early onset of these changes as shown in this thesis suggests that any fetal intervention to protect the fetal kidney from the effects of bladder outflow obstruction may need to be instituted very early in gestation