GTP-Cyclohydrolase (GTP-CH) is the rate-limiting factor in the production of tetrahydrobiopterin, a required cofactor for the three aromatic amino acid hydroxylases and nitric oxide synthases. The gene encoding GTP-CH is transcribed at high levels in infective third larval stages of a number of parasitic trichostrongylid nematodes. Here, we explored the potential role of GTP-CH within the processes of nematode development and environmentally-induced hypobiosis. For two species of parasitic nematode that are of major economic and welfare importance to livestock in temperate regions, Teladorsagia circumcincta and Dictyocaulus viviparus, we have demonstrated that each of the pre-parasitic larval stages transcribe high mean levels of cat-4 (the gene encoding GTP-CH). Using quantitative real-time polymerase chain reaction analysis and two different strains of D. viviparus, only one of which is capable of entering hypobiosis, we have shown that there were only minor differences between these strains in mean cat-4 transcript levels, both during the parasitic stages and during the earlier environmental lifecycle stages (L1 - L3). Taken together, these data indicate that, although both species of nematode produce high levels of cat-4 transcript in pre-parasitic larval stages, GTP-CH levels are probably not involved in the induction of parasite hypobiosis. Alternative roles for GTP-CH in larval development are discussed.