SORGHUM leaves contain the cyanoglycoside dhurrin located in the cell vacuole1, and when the tissue is crushed, hydrolysis of dhurrin by an enzyme system probably present in the cytoplasm1 results in the release of HCN. Cyanogenesis is probably a mechanism to protect plants from being eaten by herbivores2,3. It is generally assumed that this protection is conferred by the toxic effects of cyanide but the mechanism has not been fully investigated and the cyanoglycosides themselves or the other end products of hydrolysis may deter feeding. The degree of liberation of HCN in leaves is dependent on the age and variety of plant, as well as on certain environmental factors4–7. Leaves of young Sorghum are often rejected at the first bite by the graminivorous locust, Locusta migratoria8 but older Sorghum is eaten in large quantity. We show here that change in palatability is related to the rate at which HCN is released from the leaf at the time of biting.