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New algorithm for measurement of mean count rate of pulses from radiation detectors is described. The algorithm is based on the control of a bank of digital IIR filters governed by the state of mean count rate—stationary or transient. The algorithm solves successfully the compromise between the contradictory requirements for low statistical fluctuations in steady state and fast response to sudden changes of mean count rate. The design of the bank of HR filters is carried out by using the Chebyshev approximation of the second kind in order to achieve the high selectivity and high stopband attenuation applying as low as possible order of the constituent filters. All designed filters are of the second order. The static and dynamic characteristics of the new algorithm and the corresponding computational efficiency are tested and compared with those of the traditional ratemeter algorithms. The state of mean count rate, stationary or transient, is determined by a circuit for estimation of the gradient of mean count rate. The numbers of threshold levels within the adaptation logic circuit and their mutual positions have been optimized to achieve the best compromise. [ABSTRACT FROM AUTHOR]