Although parasites are reported to alter host responses to predators, little is known about the neurochemical mechanisms involved. Using an odour preference test, we examined the effects of an acute, subclinical infection with the naturally occurring, single host, enteric protozoan parasite, Eimeria vermiformis, on the responses of male laboratory mice, Mus musculus, to a predator. Uninfected mice avoided the odour of a predatory cat, spending a minimal amount of time in a Y-maze in the vicinity of the cat odour. In contrast, mice infected with E. vermiformis, spent a significantly greater amount of time in the proximity of the cat odour, showing a reduced avoidance of the cat odour and a reduction in predator-induced fear or anxiety. This was not related to augmented opioid activity and decreased pain sensitivity in the infected mice, as neither treatment with the exogenous opiate, morphine, nor restraint stress-induced augmentation of endogenous opioid activity, had any significant effects on the responses of uninfected mice to cat odour. The altered responses of the infected mice to the cat odour were reduced by peripheral administration of the gamma-aminobutyric A (GABAA) antagonists, bicuculline and picrotoxin, but were not significantly affected by either the benzodiazepine antagonist, Ro 15–1788, the opiate antagonist, naloxone, or the excitatory amino acid, N-methyl-D-aspartate (NMDA) antagonist, MK-801. These results indicate that infection with E. vermiformis in mice reduces the avoidance of predator odour through neurochemical systems associated with anxiety involving, at least in part, GABAA receptor mechanisms.