It is generally accepted that pheromones act by stimulating of the dendritic receptors of the olfactory neurones massed in the olfactory epithelium. This study was designed to ascertain whether it is possible for the boar pheromone androstenol (5alpha-androst-16-en-3-ol) to be transported from the nasal cavity of anaesthetized gilts to the brain and hypophysis via local transfer from the blood in the perihypophyseal vascular complex. The experiment was performed on days 18-21 of the porcine oestrous cycle (crossbred gilts, n = 6). Tritiated androstenol (3H-A; total amount 108 d.p.m. (758 ng)) was applied for 1 min onto the respiratory part of the nasal mucosa, 4-6 cm from the opening of the nares. Arterial blood samples from the aorta and from the carotid rete were collected every 2 min during the 60 min period following administration of the steroid. Total radioactive venous effluent from the head was removed and an adequate volume of homologous blood was transfused into the heart through the carotid external vein. At the end of the experiment gilts were killed and tissue samples of the hypophysis and some brain structures were collected to measure radioactivity. In addition, corresponding control tissues were collected from three untreated gilts and from three heads of gilts 60 min after 3H-A was applied post mortem into the nasal cavity. The concentration of 3H-A was significantly higher (P < 0.0001) in the arterial blood of the carotid rete than that of aorta. The mean rate of 3H-A counter current transfer from venous to arterial blood in the perihypophyseal vascular complex, expressed as the ratio of the 3H-A concentration in arterial blood of the carotid rete to the 3H-A concentration in blood sampled simultaneously from the aorta, was 1.96 ± 0.1. The concentration of 3H-A in plasma from the venous effluent from the head ranged from 1.3 to 1.8 pg ml-1. During the 60 min period of the experiment, 0.68 % of the total applied dose of 3H-A was resorbed from the nasal cavity into the venous blood. Moreover, we found that 3H-A was present in the olfactory bulb (P < 0.01), amygdala, septum, hypothalamus, adenohypophysis, neurohypophysis (P > 0.05) and perihypophyseal vascular complex (P < 0.01). These results demonstrate that, in anaesthetized gilts, the boar pheromone androstenol may be resorbed from the nasal mucosa, transferred in the perihypophyseal vascular complex into arterial blood supplying the brain and hypophysis, and then arrested in the hypophysis and certain brain structures. We suggest that in addition to the standard neural pathway for signalling pheromones, another pathway exists whereby androstenol, as a priming pheromone, may be resorbed from the nasal cavity into the bloodstream and then pass locally from the perihypophyseal vascular complex into the arterial blood supplying the brain and hypophysis, thus avoiding the first passage metabolism in the liver.