Urea kinetics were measured on two separate occasions in five adults with normal haemoglobin genotype (HbAA) and in four who were homozygous for sickle cell disease (HbSS). Prime/intermittent doses of [15N15N]urea were given orally on one occasion and intravenously on the other. In three of the nine individuals there appeared to be significant hydrolysis of the oral dose of urea before absorption, leading to spurious results for the urea kinetics. When only the studies in which isotope was given intravenously were considered, there was a difference in the rate at which urea-N was salvaged, with more urea-N being salvaged by HbSS subjects than HbAA. It is concluded that the oral presentation of isotope can be used to measure urea kinetics provided care is taken to exclude those subjects who are likely to display upper intestinal hydrolysis, and that there are differences in aspects of urea kinetics between HbAA and HbSS which may be of metabolic importance.