there is disclosed an apparatus for the selective detection and controlled gain of intranuclear and intracellular potentials . such apparatus contains an amplifier with a gain control , such amplifier having an input terminal and an output terminal and several stages each containing an integrated active electronic device . the input terminal is the input terminal of the integrated active device of the first stage , such input terminal being directly coupled by a double screened lead with a metallic microelectrode for collecting intracell and intranuclear biopotentials . the apparatus further includes a circuit for isolating the reference point of the first said stage , as compared to the null point of the power supply of the said apparatus , so that the reference point of the first stage floats according to the bioelectric signal applied at the input of the first stage , and a circuit for neutralizing the offset voltages in the stages of said amplifier , thus obtaining a good equality between the floating potential of the reference point of the first stage and the potential applied at the input terminal of the same stage . in a preferred embodiment the amplifier is directly coupled with the input terminal of a variable time constant integrator , and the stages of the amplifier are directly coupled without the use of any capacitors , thereby obtaining an improved signal to noise ratio .