Immature female rats were required to run for prolonged periods of time to obtain food. The amount of food they earned was adequate for full pubertal development and moderate growth under nonworking conditions, but both processes were blocked by the exercise requirement. Prolonged exercise also blocked the pulsatile release of luteinizing hormone (LH); only two LH pulses were seen in seven exercising females during a total of 24 h of monitoring at 8 wk of age. By comparison, almost 1 pulse/h was seen in postpubertal, normally growing females of this same age during metestrus. When the exercising females' running requirement was relaxed at 8 wk of age they experienced rapid catch-up growth and reproductive development. Both basal secretion and LH pulse frequency increased markedly within 48 h, and most of these females ovulated during the third dark period after relaxation. Altogether, the experimental paradigm and techniques employed here yield highly predictable results, and they should prove useful for exploring other neuroendocrine pathways through which excessive exercise antagonizes reproduction.