Glutamine and proline were found to be the major amino acids in the haemolymph and muscle tissue of resting adults of Periplaneta americana (L.). When the insects were forced to be active or were electrically stimulated, the level of proline fell by 50-60%, that of alanine rose by the same amount, that of glutamine rose by 30% and that of glutamate fell by 50%. Injection of large amounts (500-5000 mu g) of glutamate also resulted in a 50% fall in the level of proline. This is in agreement with previous studies indicating that glutamate may act as an excitatory transmitter in insects. The results also show that proline is used as a reserve energy store and as a source of glutamate.