Small cylindrical p-i-n semiconductor radiation detectors have been fabricated for use with internally administered radioisotope tracers and for internal dose measurements. The p-i-n semiconductor (1–4) radiation detector, because of its wide depletion region, has adequate sensitivity for either beta or gamma radiations. For this application, such detectors have been fashioned into cylinders having both diameter and height of 2 mm. Increased sensitivity may be achieved by increasing the height of the cylinder. The cylinders have been incorporated into polyethylene 4-foot catheters with an outside diameter of 3 mm.; these are sealed at the detector end and have standard microdot connectors at the other end. The connectors can be joined directly to BNC outlets through the use of adapters. Two types of catheters have been built. In one, only the p-i-n detector is incorporated in the catheter, while in the second, a microminiaturized emitter-follower and preamplifier are also incorporated. The present configuration of the electronics has a diameter of 4.1 mm., which can easily be reduced by a factor of two. The length of the emitter-follower is 5.3 mm. and that of the two-transistor preamplifier is 2.2 cm. The advantage gained through the use of the microminiaturized electronics is that a greater signal pulse height is achieved, thereby increasing the absolute sensitivity of the probe. The response of the detectors to radiation has been recorded both by counting individual pulses and by recording the change in dark current when operated with a back bias. The pulses recorded when exposed to a Co60 or Cs137 beam have a height of about ten times those due to noise in the case of the detector only. The increase in dark current produced when a back bias of 10 v is applied is 0.1 microampere for an approximate dose rate of 100 r/minute air. Through the use of a potentiometer circuit the dark current may be balanced out and the system may then be used for very low dose rates. Of course, pulse counting gives the ultimate in sensitivity, but, due to the situation of detector or electronics, one is limited in the range of high dose rates to about 1 r/minute. The magnitude of the back current is linear with dose rate in the range of 1 to 100 r/minute when a 100 kvp x-ray beam (h.v.1. = 1.5 mm. A1) is used.