The effect of intracerebroventricular injection of a nitric oxide (NO) donor (sodium nitroprusside, SNP) or NO synthase inhibitor (N ω nitro-l-arginine, l-NA) on Fos-like immunoreactivity (FLI) in the arcuate nucleus (ARC) and the medial, cortical and central amygdaloid nuclei was examined in conscious and urethane-anesthetized lactating rats. In conscious dams, the number of FLI-cells in the medial and cortical amygdaloid nuclei were significantly larger after the injection of l-NA than after SNP injection. In anesthetized dams, the number of FLI-cells was decreased in the medial and cortical amygdaloid nuclei by l-NA injection, whereas the numbers were increased in the ARC and the medial amygdaloid nuclei after the injection of SNP. The nucleus-specific response to SNP or l-NA suggested possible involvement of NO in modulation of dynamics of neuronal activity within these nuclei.