Pubertal and young adult male rats release more luteinizing hormone (LH) in response to luteinizing hormone releasing hormone (LHRH) if they are pretreated with LHRH than if they are pretreated with saline. Immature male rats do not show this self-priming effect. To examine some of the possible causes of this difference, immature or pubertal male rats were anesthetized with ketamine HCl and received 3 i.v. injections of 10 ng/100 BW LHRH or saline at 30-min intervals (Times 0, 30 and 60 min) and they were decapitated at Times 0, 15, 30, 45, 60, 75 and 90 min. Serum and pituitary LH and serum testosterone and androstenedione were determined by radioimmunoassay. A self-priming effect was apparent in pubertal rats. LH levels in response to the third LHRH injection were significantly higher compared to the response to the first injection. No self-priming effect was evident in immature rats. No changes in pituitary LH that could account for this difference were apparent. Serum testosterone levels in response to the LHRH priming were relatively higher in immature rats than in pubertal animals at 30, 45 and 60 min. Serum androstenedione levels were relatively higher in pubertal rats at 45 min. These data indicate that LHRH self-priming effect can be demonstrated in male rats using serum collected from decapitated animals for hormone analysis. They also suggest that a different relative response of serum androgens may be one cause of the appearance of LHRH self-priming during the sexual maturation of the male rat.