The effects of acupuncture on functional regulation of biological systems have been studied extensively. The most interesting feature of acupuncture is the bidirectional regulatory ability to restore homeostatic values of body systems from either up or down states

which was found by Chinese biomedical researchers several decades ago

and has recently been observed by peers internationally. Acupuncture can produce two physiological response patterns

an immediate short-term response or/and a long-lasting response. The bidirectional regulatory effects of acupuncture are rarely observed under physiological states(normal homeostasis)

rather

they are usually observed under pathological states(abnormal homeostasis)with long-lasting response patterns. This suggests that the effects of acupuncture under pathological states are different from the mechanisms under normal physiological states. This is important for understanding and developing acupuncture therapy

and also gives insight into understanding the biological control process itself.The present paper reviews the experimental literature and discusses the four conditions that produce bidirectional regulation of acupuncture

i. e. the inherent properties of the target systems

the functional states of the target system

the acupoint characteristics

and the stimulation parameters. Finally

two hypotheses are proposed to explain the mechanisms of bidirectional effects of acupuncture. The possible peripheral mechanism is that acupuncture can stimulate different types of peripheral nerve fibers to produce opposite regulatory effects;and the possible central mechanisms is that acupuncture might activate the adaptive control process of the central nervous system to restore homeostatic balance.