Contamination of aquatic environments by petroleum and its products (e.g. gasoline) is a hazard for aquatic organisms as a result of the potential toxicity of monocyclic aromatic hydrocarbons (BTEX) and polycyclic aromatic hydrocarbons (PAH). Our goal was to evaluate the acute effects of the water-soluble fraction of gasoline (WSF) on nitrogen excretion, osmoregulation, and metabolism of goldfish Carassius auratus. We first chemically characterized the WSF and then tested its effects on these physiological aspects of C. auratus, in several different exposure scenarios (0, 0.25, 5, 10 and 25% of WSF). The WSF contained high concentrations BTEX (toluene 70% and benzene 17%) relative to PAH (<1%), and low levels of several metals (Al, Fe, Zn, Sr). Routine O uptake rate (MO) of goldfish was inhibited by exposure to 5% WSF, and during post-exposure recovery, MO increased in a dose-dependent fashion. Ammonia excretion was not affected by exposure to WSF, but urea-N excretion increased progressively with the WSF concentration. The same pattern of dose/response was observed for net Mg loss rates and steadily increasing plasma lactate concentrations. Loss rates of Na, Ca, K and Cl, and plasma concentrations of Mg and urea-N were not significantly altered. We propose that acute exposure to WSF inhibits aerobic metabolism and activates anaerobic metabolism, breaking down ATP such that bound Mg is liberated and the purine ring component is metabolized to urea-N, both of which are subsequently excreted.