Acupuncture is part of Traditional Chinese Medicine, a system with an empirical basis which has been used in the treatment and prevention of disease for centuries. A lack of scientific studies to prove or disprove its claimed effects led to rejection by many of the western scientific community. Now that the mechanisms can be partly explained in terms of endogenous pain inhibitory systems, the integration of acupuncture with conventional medicine may be possible. Its use for pain relief has been supported by clinical trials and this has facilitated its acceptance in pain clinics in most countries. Acupuncture effects must devolve from physiological and/or psychological mechanisms with biological foundations, and needle stimulation could represent the artificial activation of systems obtained by natural biological effects in functional situations. Acupuncture and some other forms of sensory stimulation elicit similar effects in man and other mammals, suggesting that they bring about fundamental physiological changes. Acupuncture excites receptors or nerve fibres in the stimulated tissue which are also physiologically activated by strong muscle contractions and the effects on certain organ functions are similar to those obtained by protracted exercise. Both exercise and acupuncture produce rhythmic discharges in nerve fibres, and cause the release of endogenous opioids and oxytocin essential to the induction of functional changes in different organ systems. Beta-endorphin levels, important in pain control as well as in the regulation of blood pressure and body temperature, have been observed to rise in the brain tissue of animals after both acupuncture and strong exercise. Experimental and clinical evidence suggest that acupuncture may affect the sympathetic system via mechanisms at the hypothalamic and brainstem levels, and that the hypothalamic beta-endorphinergic system has inhibitory effects on the vasomotorcenter, VMC. Post-stimulatory sympathetic inhibition which proceeds to a maximum after a few hours and can be sustained for more than 12 hours, has been demonstrated in both man and animals. Experimental and clinical studies suggest that afferent input in somatic nerve fibres has a significant effect on autonomic functions. Hypothetically, the physiological counterpart lies in physical exercise, and the effect can be artificially reproduced via various types of electrical or manual stimulation of certain nerve fibres.