The use of intramuscular injections of procaine penicillin G (bensylpenicillin procaine) in the horse is now and then associated with acute adverse reactions (penicillin-chock). The etiology is not yet clearly understood, but the theories are several. One possibility is that it can be caused by procaine toxicity. Procaine penicillin G is a salt which is quickly dissolved in plasma. Procaine is then metabolized by plasmaesterases to non-toxic metabolites, p-aminobenzoic acid (PABA) and diethylaminoethanol.