Parasitism is essentially an ecological problem, i.e. the relationship of an organism to its environment, only in this case the environment is the body of another animal. What we have been trying to investigate is the metabolism of a parasite within its host. However, the metabolism of an infected host is not the same as an uninfected host, and the metabolism of a parasite in vivo is different from that of a parasite in vitro. There is a dynamic equilibrium between a parasite and its host and this steady state is easily disturbed by experimentation. Ideally, the host-parasite complex should be considered as a whole and not each part in isolation. Experimentally, this has not often proved feasible.