Rats fed a 6% protein diet for 14 weeks showed a normal glucose tolerance but failed to release immunoreactive insulin (IRI), following iv glucose. Furthermore the isolated perfused pancreas of the rats fed the 6% protein diet secreted eleven times less IRI than the isolated pancreas prepared from the rats reared on an 18% protein diet when both were subjected to the same glucose challenge. This difference occurred despite the fact that the total IRI content of the pancreas' was not significantly different between the two groups. In the protein-malnourished rats it is suggested that the normal glucose tolerance response, despite the failure to release a concommitant amount of IRI, may indicate increased sensitivity to IRI or may be due to the secretion in these animals of a biologically active insulin, which shows no immunological cross-reactivity with pancreatic insulin. Alternately there may be increased peripheral glucose utilisation in the low protein rats.