Reptiles have a metanephric kidney, which seperates the ducts that lead to the cloaca. Reptiles don't get rid of an excessive amount of wastes, compared to the mammals, because their glomeruli is small. Their urine is lead to the terminal portion of the alimentary canal, also known as the cloaca. From the cloaca it is disposed of along with the feces.

ReptileKidney.gif
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The nitrogenous waste of a reptile is excreted as uric acid. Reptiles do not hold a lot of urine because of their small glomerulis, and their urine is not highly concentrated, especially compared to mammals. However, it is true that their urine is often thick with crystaks of uric acid.

Sources:

"excretion." Encyclopædia Britannica. Encyclopædia Britannica Online School Edition. Encyclopædia Britannica, 2010. Web. 27 May 2010 <http ://school.eb.com/eb/article-58714>.