The effect of NaCl in maturation media on histone H1 kinase activity of porcine oocytes was examined at the end of culture for maturation. Oocyte-cumulus complexes were cultured in modified Whitten's medium containing different concentrations of NaCl (44.50, 68.49, 92.40, 116.40 or 140.35 mM). The media were supplemented with 10% porcine follicular fluid and hormones for 20 h and then without hormonal supplements for an additional 20 h. At the end of culture, oocytes were sampled for morphological observation, for glutathione assay and for histone H1 kinase assay. At the end of culture, the incidence of oocytes at each meiotic stage was not different among groups. However, histone H1 kinase activity and glutathione content were higher (P < 0.01) in oocytes cultured in media with lower NaCl concentrations. Histone H1 kinase activity in the oocytes at the end of maturation culture was correlated with intracellular glutathione content (r = 0.899, P < 0.01). These data indicate that histone H1 kinase activity and glutathione content of porcine oocytes at the end of culture for maturation are reduced when a high salt medium is used for maturation.