Dimethyl sulfoxide (DMSO) is a very simple compound that has stimulated much controversy in the scientific and popular literature. Fig. 1 It is an aprotic solvent. Therapeutic and toxic agents that are not soluble in water are often soluble in DMSO. DMSO has a very strong affinity for water; on exposure to air, pure DMSO is rapidly diluted. DMSO's physiologic and pharmacologic properties and effects are incompletely understood. Properties that are considered to be particularly important to its therapeutic and toxic effects include: its own rapid penetration and enhanced penetration of other substances across biologic membranes; free radical scavenging; effects on coagulation; anticholinesterase activity; and DMSO-induced histamine release by mast cells. DMSO's systemic toxicity is considered to be low. Combinations of DMSO with other toxic agents probably constitute its greatest toxic potential. The scientific literature is reviewed with particular attention to mechanisms underlying DMSO's reported therapeutic and toxic effects. Currently approved, veterinary applications of DMSO are limited. DMSO's potential value in specific, approved and unapproved veterinary applications is discussed.