The 20 month-long treatment of female rats with daily doses of 0.1 or 0.5 mg of polypeptide pineal extract (PPE) per animal increased their lifespan by 10 and 25%, respectively, as compared with controls. At the age of 16--18 months, 38% of control rats exhibited persistent disturbances in estral function (constant estrus or repeated pseudogestations), whereas these disorders were observed in 7% of experimental animals only. After administration of PPE to 16--18 month-old female rats checked for sterility by a two-week mating, a second mating period resulted in gestation development in four out of 16 animals and deliveries, accordingly. While chronic treatment with PPE did not affect the rate of neoplasm incidence, the mean age of tumour detection in the control group was 697 days and in experimental groups it was 811 and 868 days, respectively. Certain aspects of the interrelationship of rate of ageing, lifespan and specific age pathology are discussed.