Although acetylcholinesterase inhibitors are accepted antagonists of nondepolarizing neuromuscular blockade, many basic questions are still unanswered. What is the relationship between receptor occupancy and adequate ventilation? What are the effects of changes in acid-base balance and temperature? What are the mechanisms of the various antibiotic-induced neuromuscular blockades, and what antagonizes them? This review is an attempt to summarize the known factors infuencing relaxant blocks and to identify the unknown factors. The need for further studies is obvious.