Neurospora crassa has a putative histidine phosphotransfer protein (HPT-1) that transfers signals from 11 histidine kinases to two putative response regulators (RRG-1 and RRG-2) in its histidine-to-aspartate phosphorelay system. The hpt-1 gene was successfully disrupted in the os-2 (MAP kinase gene) mutant, but not in the wild-type strain in this study. Crossing the resultant hpt-1; os-2 mutants with the wild-type or os-1 (histidine kinase gene) mutant strains produced no progeny with hpt-1 or os-1; hpt-1 mutation, strongly suggesting that hpt-1 is essential for growth unless downstream OS-2 is inactivated. hpt-1 mutation partially recovered the osmotic sensitivity of os-2 mutants, implying the involvement of yeast Skn7-like RRG-2 in osmoregulation. However, the rrg-2 disruption did not change the osmotic sensitivity of the wild-type strain and the os-2 mutant, suggesting that rrg-2 did not participate in the osmoregulation. Both rrg-2 and os-2 single mutation slightly increased sensitivity to t-butyl hydroperoxide, and rrg-2 and hpt-1 mutations increased the os-2 mutant's sensitivity. Although OS-1 is considered as a positive regulator of OS-2 MAP kinase, our results suggested that HPT-1 negatively regulated downstream MAP kinase cascade, and that OS-2 and RRG-2 probably participate independently in the oxidative stress response in N. crassa.