Rainbow trout (Oncorhynchus mykiss) were exposed to 1.65 microM of waterborne copper for 24 h. Fish were then transferred to metal-free water. Metallothionein mRNA induction in rainbow trout liver and gill tissue, hypoxia-inducible factor-1 (HIF-1alpha) accumulation in gill tissue and arithmetic mean thickness of gill epithelium (Har) were determined at 4 and 24h of exposure as well as 48 h after transfer to metal-free water. The arithmetic mean distance from water to blood was significantly elevated after both 4 and 24 h of exposure (Har was 4.67 and 4.66 microm, respectively in exposed fish, compared to 3.81 and 3.62 microm for the corresponding control fish). During the 48 h recovery Har returned towards the control values; the recovery value of 4.21 microm was significantly lower than values during exposures. There was also a significant increase in gill metallothionein mRNA levels after the 4 h exposure with MT/GAPDH ratio of 1.288 versus the control value of 0.988. In liver, metallothionein induction was not observed. HIF-1alpha protein showed an increased accumulation in gills after 4 h, with the HIF-1alpha/alpha-tubulin ratio of 0.562 being significantly higher than the 24 h exposure value of 0.232. These results suggest that exposure to copper for four hours causes hypoxia in the gill epithelium, which is adequate for the activation of HIF-1alpha.