The rat with portocaval anastomosis represents a convenient took for the investigation of (a) factors determining the availability of tryptophan to the brain and (b) the role, if any, that altered tryptophan metabolism may have in the development of hepatic encephalopathy. It was found that increases in brain tryptophan following anastomosis paralleled plasma free tryptophan rather than plasma total tryptophan regardless of whether or not correction for inhibition from amino acids competing with tryptophan for uptake into brain was applied. Nevertheless, while plasma free tryptophan exerts a major influence on brain tryptophan in both sham-operated and anastomosed rats, in the latter group brain tryptophan is raised further by some other mechanism. The anastomosed rat was also found to be behaviourally abnormal in a number of test situations. Thus, they were hypoactive during chronic exposure to an open-field and were less responsive to electric shock. Following the administration of tryptophan, sham-operated rats were also less active in an open-field and less responsive to electric shock when compared with saline-treated rats. Thus, anastomosed rats have behavioural abnormalities for which altered tryptophan metabolism might, to some extent, be responsible.