A definition is proposed for biological ("internal") time tau(t) for a growing organism whose weight variation obeys the law w(t): tau(t) = 1/c(w) (t) = w(t)/w'(t), where t is physical ("external") time, w'(t) is weight increase rate, and c(w) (t) = w'(t)/w(t) is specific growth rate. Properties of functions tau(t) and w(tau) were studied for those cases when growth curves w(t) were described by Bertalanffy's or logistic equations.