The results of 24 h food preference tests have suggested that learned food aversions may be involved in the development of anorexia in tumour bearing rats and in patients with cancer. We have performed similar tests over longer periods, up to 10 days, in male rats implanted with Leydig cell tumours, using semisynthetic diets containing differing proportions of fat, protein and carbohydrate. Tumour growth caused anorexia (16-30% decrease in food intake) and cachexia (78% decrease in body fat and 18% decrease in body protein, but 16% increase in body water). Both tumour bearing and control rats preferred a high carbohydrate diet to a high fat diet regardless of their previous diet: tumour bearing rats showed no evidence of a learned food aversion in these experiments. Tumour bearing rats did show an initial preference for a novel high protein diet when this was offered as an alternative to the normal protein diet they had previously been consuming, but this apparent learned food aversion disappeared on the second day of the test and was in fact reversed on all the subsequent days of the test. However, tumour bearing rats did show a sustained preference for a novel low protein diet when this was offered as an alternative to the normal protein diet they had previously been consuming. These results suggest that anorexia in the tumour bearing rats was not caused by a learned food aversion. However the results do indicate that the tumour bearing rats may have developed a specific aversion to protein in the diet. Leydig cell tumours are known to secrete large amounts of oestradiol. However injections of oestradiol in normal male rats caused an increase in body fat content and had no effect on the rats' preference for dietary protein. Clearly hypersecretion of oestradiol was not responsible for the loss of body fat, the fluid retention and the aversion to dietary protein which characterised the tumour bearing rats. The mechanisms by which tumour growth causes anorexia and cachexia in these rats remains obscure.