The osmoregulation of the nauplius of the brine shrimp, Artemia salina, was investigated using micropuncture and microanalytical techniques. The naupliar body fluid, hemolymph was hyposmotic to and had lower Na concentrations than the suspending medium for the range of medium salinities from 80 to 4,900 mM NaCl. In medium containing 20 mM NaCl, the hemolymph was hyperosmotic to the medium, with osmolarity of 101 +/- 8 mosmol/1 and with [Na] of 49 +/- 11 meq/1. Whereas the maximal observed NaCl concentration gradient between hemolymph and medium was 4,785 mM, during the incubation of nauplii in artificial seawater (osmolarity: 932 mosmol/1; and [Na]: 502 meq/1) the osmolarity and [Na] of the naupliar hemolymph were 161 +/- SD 16 mosmol/1 and 86 +/- 14 meq/1, respectively. The influx and efflux of Na between medium and hemolymph were measured using 22Na. The fluxes of this ion were temperature dependent. The main site of efflux of 22Na was the neck organ as was shown by experiments of differential recovery of 22Na introduced in the hemolymph. These studies demonstrate that the nauplius of A. salina has the ability to osmoregulate not only against high environmental salinities but also against low salinities approaching those of freshwater.