Ultimate heat-transfer limitations imposed by sonic vapor flow were determined in heat pipes for sodium, potassium, and cesium working fluids. Each fluid was investigated in a heat pipe consisting of an inner porous tube, an annulus for liquid return, and an outer container tube. Thin, rigid tubes with very small pores were obtained by compressing several layers of fine-mesh screen. These tubes allowed large capillary forces to develop so that sonic vapor flow could be achieved at several operating temperatures. The results of the investigation showed that sonic limitations were influenced strongly by the temperature and the working fluid. Reasonable agreement was found between the experimental results and existing theory. It was also found that the theory could be used to predict evaporator pressure and temperature gradients when the heat pipes were operated at various fractions of their ultimate heat-transfer capability.