The age-dependency of isoproterenol (IPR) induced DNA synthesis was investigated in different organs of Balb/c mice. Although modifications of the physiological rate of DNA synthesis after IPR injection occurred also in liver and spleen, a quasi-linear decrease of the peak of IPR response with advancing age was observed only in submandibular glands. Such a decrease was observed when animals were injected with 10(-4) g IPR per g body weight, lower doses being unable to discriminate between young and old mice. In spite of some differences between mice and rats, the early appearance and linearity of the age-dependency remain common features of IPR response.