It is uncertain whether the beneficial effects of the high-fibre, high-carbohydrate diets generally recommended for improved glucose control in diabetic patients are in fact due to the fibre. In this study the effect of the long-term ingestion of the dietary fibre, konjac - glucomannan (K-GM), on glucose tolerance and immunoreactive insulin (IRI) concentrations was investigated in normal baboons and compared with the short-term effect of the concurrent feeding of K-GM and glucose. Eighteen young baboons were divided in a control and a K-GM group. Glucose tolerance tests ( GTTs ) performed at baseline showed that the two groups were strictly comparable. The K-GM group then received the same diet as the control group plus 5% K-GM. After a period of 12 - 14 weeks on the diets, the K-GM baboons had significantly higher fasting blood glucose and serum IRI values. The results of GTTs showed, however, that long-term feeding of K-GM improved glucose tolerance significantly and decreased the area under the IRI response. These long-term effects were more pronounced than the short-term effects. It is concluded that the long-term ingestion of K-GM per se has beneficial effects on glucose homeostasis in normal baboons and that the possible mechanisms by which these effects are established warrant further investigation.