The invention provides methods of screening compounds for potential pharmacological activity using nematode worms, principally but not exclusively the nematode C. elegans. Specifically, the invention relates to the use of nematodes modified to have certain characteristics which provide advantages for compound screening, such as constitutive pharyngeal pumping, increased gut permeability or altered gut molecular transport. Methods for selecting suitably modified nematodes from a population of nematodes are also provided.