Lipoic acid (LA) is a drug with proven effect on pathogenetic mechanisms of diabetic polyneuropathy. Recent studies have indicated that LA can also be useful in the treatment of other disorders, including cardiologic, neurological, dermatologic and even oncologic diseases. However, the mechanism of so wide spectrum of pharmacological activities of LA is still insufficiently known. The aim of the present study was to broaden our knowledge of the mechanisms of biological action of LA delivered from exogenous sources. One of difficulties in studying the mechanisms of pharmacological action of LA is related to the fact that the effects of its action in vitro can differ from those observed in vivo. Nonetheless, in spite of many questions that still remain unanswered, the results of the present study unequivocally indicate that one of the mechanisms underlying biological activity of LA involves its participation in metabolic transformation of sulfane sulfur and hydrogen sulfide, defined as reactive sulfur species (RSS). It should be remembered that multidirectional regulatory functions of RSS are manifested mostly by changing the activity of many enzymatic and receptor proteins. Therefore, the hypothesis explaining LA efficacy in so many often remote pathological states by its effect on RSS transformations seems reliable and logically coherent.