A method has been developed which permits continuous recording of intratubular pressures and simultaneous infusion of test solution at rates of 0–50 nl/min through a single microcapillary. Design and operation of the system is described which consists of an ultraminiature pressure transducer, a micro-perfusion pump and the probing microcapillary. Sealing of the latter was achieved by a screw collar which permits rapid replacement of capillaries without deleterious pressure spikes. At optimal supply voltage the recorded signal was independent of input voltage changes (up to ± 100 mV) and directly proportional to a) pressure applied to the microcapillary tip and b) flow rate through the capillary. — As example the pressure increments following local infusion into the proximal convolution of indigocarmine stained saline or reabsorption inhibitors at a rate of 10 nl/min was measured. During the infusion of 10% mannitol intratubular pressure rose to a plateau and remained above control levels. Application of potassium cyanide (1.5×10−10 M/min) resulted in a gradual pressure increase starting at the onset of the intratubular perfusion. A qualitative similar rise was observed during administration of furosemide (2.2×10−11 M/min), however the effect exhibited a time delay of several minutes.