Caffeic acid (CA) and Trolox are phenolic acids that have beneficial antioxidant effect, but the underlying mechanisms involved are not fully understood. The extent to which CA and Trolox protect against sodium nitroprusside (SNP)-induced oxidative cell injury was investigated in cultured rainbow trout gill cells. The cells exposed to SNP for 24 h displayed a dose-dependent leakage of lactate dehydrogenase (LDH) and decreased cell viability as indicated by the MTT assay (mitochondrial dehydrogenase activity). Both effects were prevented by treatment with 50 microM CA or Trolox. CA or Trolox, protected against SNP-induced caspase-3 activation and DNA fragmentation, indicating a reduction of apoptosis. Thus, the results indicate that SNP induced cell death is caspase-3 related apoptosis and the treatment with CA inhibited the apoptotic pathway. In addition, we studied the effect of CA and Trolox on expression of zinc-responsive antioxidant genes such as metallothioneins (MT), glutathione-S-transferase (GST Class pi) and glucose-6-phosphate dehydrogenase (G6PD) in cultured gill cells. CA, 100 microM, increased accumulation of mRNA for MTA, MTB, GST and G6PD in cells. Thus, in addition to its ability to sequester free radicals, CA may protect against oxidative stress through expression of zinc-induced antioxidant proteins. Because of these properties we suggest that CA could be a beneficial additive to fish feeds in aquaculture.