The hormone induction of vitellogenin (Vg) gene expression was investigated in primary cultures of immature male rainbow trout hepatocytes. Vg cDNA was cloned from the liver of a female rainbow trout injected with estradiol-17beta (E2). A Vg mRNA of about 6.6 kb was detected in the hepatocytes after stimulation with 2 x 10(-9) M E2 for 24 h. The intensity of this signal increased with E2 concentration. Several other hormones including progesterone and androgens (testosterone, androsterone, methyltestosterone) also induced Vg mRNA not only at high (2 x 10(-5) M) but also at low (2 x 10(-9) M), physiologically relevant steroid concentrations. Only cortisol failed to do so at any tested concentration but, surprisingly, like progesterone, it induced an additive effect in the presence of E2. On the other hand, tamoxifen drastically reduced the expression of Vg gene whether stimulated by E2 or any other tested steroid. In conclusion, our studies have highlighted the unusual specificity of Vg mRNA induction in fish.