Specimens are placed in sealed chambers to which scintillation cells are attached and radon concentrations,R, are measured in the air of the chambers at intervals thereafter by inserting the cells into scintillation counters. The Rn emanation rate is calculated from the rate at whichRincreases, or from the maximumRreached. Uncertainties are estimated to be 10%–25%, detection limits 10−19Ci cm−2s−1or 5×10−19Ci g−1s−1. Effects of disequlibrium in the Rn decay chain, or the diffusion rate of Rn, and of Rn leakage are considered. Powdering a rock usually has little effect on its emanation rate per gram.