A heat pulse generated inside a needle probe can be used to measure the thermal conductivity of surrounding rock fragments or drill cuttings. As the pulse of heat is conducted outwards into the surrounding aggregate of rock fragments and water, the decrease in temperature inside the probe is recorded as a function of time. An asymptotic relation between probe temperature, conductivity, and inverse time since the heat pulse is shown to be accurate for the range of times used. The relatively slow thermal response of the probe in samples with higher conductivities is accommodated by a delay in the origin time of the pulse. The combined correction for finite pulse length and slow probe response is shown to be small and predictable. The thermal conductivity of rock fragments is calculated from a model that is dependent on the water content of the sample, as in other methods. Results using this method and a divided bar apparatus are equivalent, given the expected accuracy of a divided bar. The measured thermal conductivities of water, fused quartz, and crystalline quartz compare closely with their accepted values.