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Down-hole geothermal instrumentation must operate over a large temperature range. The technology and capabilities of room temperature to 300°C hybrid and printed-circuit (PC) board electronics that were developed during the last two years to meet that need are summarized. To ensure rapid widespread commercialization, this technology was developed, insofar as possible, using commericaUy available components, devices, and materials. Initial extensive high-temperature characterization revealed that selected thickfilm passive components and silicon junction-field-effect transistors had electrical parameters sufficiently insensitive to temperature change and sufficiently constant in time at high temperatures to form the backbone of this circuitry. Attachment techniques needed to be developed, since standard methods failed at high temperatures. Similarly, circuit design innovations were needed because of the restricted list of parts. Voltage regulators, line drivers, voltage comparators, special purpose amplifiers and multiplexers were constructed and operated over the 25-300°C temperature range. Temperature and pressure monitoring instruments using these circuits have been used for downhole measurements in geothermal wells. Methods of fabrication, circuit performance, and the scope of future work are discussed.