A boronimeter capable of rapid and accurate determination of the boron concentration in the primary coolant of pressurized water reactors is described. The instrument utilizes the neutron transmission method and is thus responsive to the 10B content of the primary coolant. A unique feature of the boronimeter is a servo-driven absorber sleeve that is positioned to compensate for changes in the boron concentration. This null-type operation is well suited for on-line analyses and also simplifies the calibration of the boronimeter.