Heat shock protein 70 (HSP70) expression was assessed in hepatic tissue of a marine teleost Sparus sarba after exogenous hormone administration. Using a PCR-amplified, homologous HSP70 cDNA clone, as a probe in Northern analysis, we detected a 2·3 kb transcript which was elevated after exposure to a temperature 7 C above the ambient. For our studies on hormonal effects on HSP70 expression, groups of fish were administered recombinant bream GH (rbGH), ovine prolactin (oPRL) or cortisol daily over a 7-day period. Quantification of hepatic HSP70 transcript revealed that the administration of GH and PRL significantly reduced HSP70 mRNA abundance by 42% and 54% from saline-injected fish respectively. Also hepatic HSP70 levels were reduced by 76% and 64% as determined by immunoblotting after rbGH and oPRL treatment respectively. The administration of exogenous cortisol did not alter hepatic HSP70 mRNA or protein levels in S. sarba. The results obtained in this study are the first evidence for hormonal modulation of heat shock protein expression in fish. The significance of these results is discussed within the context of current knowledge on the roles of these hormones in teleostean stress response.