Hyposensitivity to drugs induced by morphine and some related substances in the cat.

1. Hyposensitivity to noradrenaline occurred concomitantly with morphine tachyphylaxis in the absence of any obvious depressant effect on the heart or peripheral circulation.2. The actions of adrenaline, noradrenaline and angiotensin and, occasionally, other drugs were tested before and after repeated injections of morphine or one of ten related substances in various preparations of the spinal cat.3. Tachyphylaxis in response to morphine could be related to a simultaneous decrease in the tissue responses to adrenaline, noradrenaline and angiotensin but such a parallelism has not been shown for the substances related to morphine.