Electrophysiological studies demonstrated that the olfactory epithelium of mature male brown trout Salmo trutta parr was acutely sensitive to F-series prostaglandins (PGFS) PGF(1alpha) and PGF(2alpha), with detection threshold concentrations of 10(-11) M. The olfactory epithelium was also sensitive to the PGF metabolite 15-ketoPGF(2alpha) (threshold 10(-8) M), but did not detect a further metabolite, 13,14,-dihydro-15-ketoPGF(2alpha). Immature brown trout did not detect any of the prostaglandins tested. Exposure of mature male brown trout parr to waterborne PGF(1alpha) and PGF(2alpha) (concentration 10(-8) M). resulted in significant increases in levels of expressible milt and the plasma concentrations of 17,20-betadihydroxy-4-pregnen-3-one, testosterone and 11-ketotestosterone. The olfactory epithelium of both immature and mature male brown trout parr was sensitive to the urine and ovarian fluid from ovulated female brown trout. Exposure of mature male brown trout parr to ovarian fluid resulted in an increase in the levels of plasma 17,20beta-dihydroxy-4-pregnen-3-one whilst exposure to urine increased the levels of expressible milt. In addition, PGF(2alpha) was found to be present within both the urine and ovarian fluid of mature female brown trout. It is suggested that the F-series prostaglandins have a role as priming pheromones in male brown trout.