The endogenous concentrations of Rb and Cs have been determined in needles of 56 trees (Norway spruce, Picea abies) from eight different sites. Analysis was done by instrumental neutron activation. Concentrations were found to have a very large range (3-28,000 ng Cs/g and 1–190 μg Rb/g). The values on a given site have a tendency toward a log-normal distribution. There is a significant correlation between Rb and Cs, but the correlation is not linear. The concentrations of both elements are a function of the needle age. They decrease smoothly, approaching a constant value, when going from needle age class 1 to 5. It is shown that one algebraic function describes this biodynamic behavior on all sites and at all concentration levels. The function and its parameters are discussed.