Publisher Summary This chapter elaborates the catecholamine metabolism and amphetamine effects on sensitive and insensitive mice. d -Amphetamine neither increases the spontaneous motility nor does it induce the stereotyped behavior, and grouping effect in C3H mice as do the doses active in NMRI mice. d -Amphetamine does not significantly increase the body temperature of C3H mice, while it elicits a dose-dependent hyperthermia in NMRI mice. On the contrary, at low doses, it decreases the body temperature in C3H mice. d -Amphetamine decreases noradrenaline in the brain-stem, and increases homovanillic acid in the striatum, but it only slightly affects noradrenaline, and does not modify HVA concentrations of C3H mice. The thermic response to amphetamine in normal untreated mice is probably a polygenically-inherited trait. In an attempt to understand the genetically determined different reactivity to amphetamine, the basal brain concentrations of the biogenic amines and of their metabolites were compared in the two strains.