The acute effects of a 20 min constant rate intra-arterial infusion of LH and catecholamines on testicular blood flow and plasma testosterone concentrations were examined in sodium pentobarbitone anaesthetized rats. Ovine LH (2.5 microgram/min) elicited a 6-fold increase in testosterone concentration and a significant decrease of testicular vascular resistance. Noradrenaline and adrenaline (0.4 microgram/min) caused significant depressions in the plasma testosterone levels. The catecholamines induced no absolute changes in testicular blood flow but noradrenaline caused an increase in testicular vascular resistance. It was concluded that the catecholamine induced reductions in testosterone concentrations were not due to a vascular effect on the testis.