The difference in the signals returned from two tetrahedral corner reflectors 6.9km apart along the same line of sight has been used as a measure of the attenuation of 8.6mm radiation in rain. Eight recording rain gauges along the line provided an estimate of the rate of rainfall. The relationship between the attenuation and the rate of rainfall was found to be A decibels per kilometre = 0.26(5)R millimetres per hour. Mean values of A over tens of minutes ranged from 0.20R to 0.39R. The relationship A = 0.265R was used to estimate the rainfall from the measured attenuations along the path. Only six of the 23 estimates of the rainfall total fell outside the range 80% < radar rainfall/gauge rainfall < 120%. However, as a practical means of measuring rainfall, the method would be restricted to light and moderate rains, since, in heavy rain, the signal would be attenuated to below the minimum detectable level, except over very short path lengths.