The survival of 594 white rats (Rattus norvegicus)-351 females and 241 males[long dash]was followed under identical genetic and life conditions. The animals were divided into six groups: (1) females with a mean of 8.1 pregnancies; (2) females without pregnancies; (3) castrated females; (4) males with sexual activity; (5) males without sexual activity; (6) castrated males. The shortest value of 50% survival was found in females with pregnancies. In other groups of females this value was greater than for males. Age tests carried out with the collagen fibres in the tail tendons of these animals showed that females with pregnancies were biologically older than animals of other groups; this was borne out by the determination of mechanical tension due to thermic contraction as well as of tae loosely combined hydroxyproline (or collagen). It is deduced that pregnancies cause a general aging of the body collagen; this may bring about the shortening of life.