The effect of methamphetamine on the expression of interleukin-1 beta (IL-1 beta) mRNA in the rat brain was investigated with Northern blot analysis. Methamphetamine (2-15 mg/kg, i.p.) caused a marked induction of IL-1 beta mRNA in the hypothalamus among the 8 brain regions examined, in a dose-dependent manner. The level of IL-1 beta mRNA reached a maximum at 1 h and rapidly declined within 2 h after the injection of 15 mg/kg. These results provide the first evidence that methamphetamine induces the expression of IL-1 beta mRNA in the hypothalamus, which may be partly involved in the production of central actions of this drug.