In order to determine the influence of physical training on menstrual disturbances in sportsgirls, the levels of luteinizing hormone (LH), follicle stimulating hormone (FSH) and melatonin have been studied in young athletes of track and field speciality using the Cooper test. Basal hormone levels and anthropometric data were also studied in age matched control girls. No significant differences in LH, FSH and melatonin hormone concentrations were observed between the PRE and POST Cooper test. However, significantly lower basal levels of LH were found in the early follicular phase or luteal phase of sportsgirls when contrasted with the control girls. No differences in FSH levels were observed in the early follicular phase of sportsgirls but higher FSH levels were found in the luteal phase. Daytime melatonin levels of sportsgirls were significantly higher than those in control girls. Age and anthropometric parameters studied showed no differences in height, weight, tricipital skinfold and percentage of body fat, but abdominal and subescapular skinfold measures were greater in control girls than in sportsgirls. It appears that continuous physical training can produce alterations in antireproductive hormone secretion such as melatonin, which can play an inhibitory role on the menstrual cycle hormone patterns in sportsgirls.