One hundred and forty-eight growing pigs were used in four experiments conducted to determine the effect of lithium (Li) on the level of aggression among newly mixed, previously unacquainted pigs. Pits maintained on low levels of dietary Li (17 to 84 ppm) attained serum Li concentrations of .1 to .7 mEq/liter. At these low serum levels, biting activity was consistently lower than it was among control pigs, although the difference was usually not statistically significant. Low levels of dietary Li did not significantly affect rate of gain or feed efficiency. When high concentrations of serum Li were achieved by an intraperitoneal injection, pig aggression was reduced (P < .05) to almost zero for 10 hr postmixing. This effect was probably due to drug-induced malaise associated with potentially toxic serum Li concentrations (above 3.0 mEq/liter). These data suggest that Li can substantially reduce aggression among pigs that are unfamiliar with each other. However, the resulting emesis and reduced feed intake associated with high levels of serum Li are sufficient to contraindicate its use for reducing fighting among unfamiliar growing pigs.