Our hypothesis is that the endogenous opiate system is malfunctioning in autistic people. Levels of endorphins (and perhaps enkephalins) are fluctuating below and above normal levels leading to sensory deprivation. The sensations from all the sense organs are too great when the endorphin (and enkephalin) level is low making them meaningless or too little when the endorphin level is high leading to lack of sensation. Some of the bizarre repetitive behaviours are endorphin producing activities which cut down distressing sensation, others are attempts to cut down excessive stimuli from the environment, while others remain completely baffling. The thalamus could be one of the regions of the brain where inhibitory neurones and opiate receptors are malfunctioning and the feed back system initiated by the hypothalamus could have been damaged by the same agent. In both cases the damage could have been caused by chemical imbalance about the time of birth affecting the normal action of endorphins, enkephalins and prostaglandins. More boys than girls may be autistic because their brains are less mature at birth or because the male and female opiate systems are distinct.