Treatment of adult rats with 25 iu follicle-stimulating hormone (FSH) did not influence testicular blood flow, serum testosterone, vasomotion or intravascular leucocyte concentration at 6, 12 and 24 h after treatment. Treatment of adult rats with 50 iu pregnant mare serum gonadotrophin (PMSG) resulted in a two-fold increase in testicular blood flow at 24 h, and an increase in interstitial fluid volume at 36 h after treatment. This PMSG-induced increase in blood flow did not occur in Leydig cell-depleted animals, suggesting that the effect on blood flow is mediated via the Leydig cells. PMSG injection stimulated testosterone secretion but it did not influence vasomotion, and it only marginally increased the secretion of leucotactic factors in the testis. The present study suggests that FSH has no apparent effects on testicular blood flow and that the effects of PMSG (a hormone with both FSH and LH-like activity) is mediated via stimulation of the Leydig cells.