Parasitic nematodes of grazing livestock represent an increasing economic and welfare problem for British agriculture. By investigating specific life-cycle stages of these parasites, it may be possible to identify key molecules or pathways that are required for the survival of the worms, and thus exploit these for future control strategies. It has been shown previously that the third larval stages (L3) of the ovine parasitic nematode Teladorsagia circumcincta produce high levels of transcript for the enzyme GTP-Cyclohydrolase relative to later developmental stages. As the ratelimiting factor in the production of tetrahydrobiopterin, GTP-Cyclohydrolase is required for a number of different biochemical pathways, including those involved in the production of serotonin and melanin. As the L3 do not feed, it can be hypothesised that, if finite resources are being used in the production of transcript encoding this enzyme, then it may be important for survival. In this thesis, a number of approaches were taken to explore the function of GTPCyclohydrolase in the life-cycle development of T. circumcincta. The closely related parasite, Dictyocaulus viviparus, was used as a model organism to explore the role of GTP-Cyclohydrolase and serotonin production with regards to larval arrest, or hypobiosis. This process occurs readily under experimental conditions in D. viviparus, which is not possible with T. circumcincta. Quantitative PCR was used to examine GTP-Cyclohydrolase transcript levels in two different strains of D. viviparus, one that enters larval arrest when exposed to cold conditions and one that does not. No differences were observed between the two strains suggesting that GTP-Cyclohydrolase was unlikely to be involved in hypobiosis. The model nematode, Caenorhabditis elegans, was used to perform functional complementation experiments to assess the role of GTP-Cyclohydrolase in the cuticle, as it has been shown previously that C. elegans GTP-Cyclohydrolase mutants have a ‘leaky cuticle’ and are killed by lower doses of anthelmintics and bleach than the wild-type worms. The T. circumcincta gene for GTP-Cyclohydrolase was able to restore cuticular integrity of C. elegans GTP-Cyclohydrolase-deletion mutants, suggesting that the role played by the protein in both species is similar. In vitro inhibition experiments using a chemical inhibitor of GTP-Cyclohydrolase showed that T. circumcincta larval development was disrupted in the presence of the inhibitor. It was also shown that T. circumcincta L3 that were exposed to sunlight produced melanin, suggesting that the levels of GTP-Cyclohydrolase observed in the preparasitic stages of T. circumcincta may be required for the synthesis of melanin. Together, these data suggest that GTP-Cyclohydrolase is required by the preparasitic stages to survive on pasture. Ultraviolet radiation has been shown previously to be harmful to T. circumcincta L3, so if the melanin production provides protection from this, then it would be crucial for the survival of the pre-parasitic stages.