In stable and predictable environments, grazing herbivores can use spatial memory to increase foraging efficiency. As most natural systems are characterized by unpredictability in the distribution of resources, we explored the foraging efficiency of sheep, Ovis aries, in relation to variability in spatial distribution of resources, predicting that as the environment became more variable, spatial memory would be replaced by sampling behaviour. We manipulated resource distribution by using bowls of food pellets to create seven preferred resource sites within a grass pasture. The provisioning of the bowls with pellets and the mass of pellets therein were altered to create four treatments in which resources remained stable or varied within and between resource sites. Where bowl position was predictable, sheep showed increased foraging efficiency by reducing the time devoted to pellet foraging in relation to the total mass of pellets consumed and by visiting fewer empty bowls while searching within a site. Where bowl position became less predictable, sheep increased sampling within sites, visiting more empty bowls before leaving a site. However, in contrast with previous observations, sheep did not return preferentially to the highest-quality sites within the test plot. This study suggests that sheep could use spatial memory at small spatial scales to improve foraging efficiency where resource distribution is predictable. The ability to switch between foraging tactics may allow sheep to maintain foraging success under variable conditions. (c) 2004 The Association for the Study of Animal Behaviour.