Abstract 1. 1. Measurements were carried out on the excretion of NH + 4-N and total-organic-N of Penaeus japonicus exposed to different salinities ( ca. 37‰ S and 21%.S) at low (10–14°C) and high (25°C) temperatures. 2. 2. At steady state conditions NH 4 +-N excretion is higher at the lower salinities; the excretion rate is strongly dependent to temperature. Non-NH 4 +-N excretion is neither temperature, nor salinity dependent. 3. 3. After a change in salinity NH 4 +-N excretion shows a strong hysteresis; Non-NH 4 +-N excretion is temporarily increased after transfer to lower as well as to higher salinities. 4. 4. The results indicate that rapid acclimatory changes in the amino acid pools of various tissues take place by reversible synthesis/degradation processes.