When the crab Pachygrapsus crassipes is immersed in 50% or 150% sea water resulting exchanges of Na are principally between the blood space and the external medium. However, with respect to K, Ca and Mg, exchanges are mainly between an extra-vascular pool and the external medium although alteration in the blood concentrations for the 3 ions indicates that the blood also participates. When Pachygrapsus is desiccated, blood K, Ca and Mg concentrations increase more percentagewise than does Na suggesting that the first 3 ions shift from the extra-vascular pool into the blood space. This is believed an indication of physiological breakdown. Comparison of ion concentrations in the urine and in the blood of Pachygrapsus under different osmotic conditions and conditions of desiccation indicates that the antennary glands are relatively ineffective as regulators of Na, K and Ca, but are probably strong regulators of Mg.