1. A decline in the renal excretory rates of water and electrolytes occurred in the trout, Salmo gairdneri, at the time of smolting.

2. This decline appeared to be almost entirely attributable to a reduction in the glomerular filtration rate.

3. Although there was an increase in the total urine osmolal concentration as the urine flow decreased, this decrease was insufficient to maintain the total osmolal excretory rate at a constant level.

4. The sodium, potassium and chloride concentrations of the urine only accounted for approximately 50% of the observed real osmolality of the urine in both the smolting and the non-smolting fish; the difference may have been due to the presence of osmotically active nitrogenous compounds.

5. No selective change in the tubular reabsorptive or secretory processes were found to accompany the reduction in glomerular filtration at the time of smolting.

6. The urine: plasma-ion concentration ratios showed no evidence for any selective concentration in the renal tubule.

7. During the transition from the smolting to the post-smolting condition the renal excretory pattern was restored to that of the pre-smolting trout.

8. The possible significance of these changes in relation to the excretory changes known to occur during the adaptation of salmonid fishes to a marine environment are discussed.

This study was substantially completed at the University of British Columbia, Vancouver, Canada. The research was supported by grants to one of us (W.N.H.) from the National Research Council of Canada (T-167) and the National Science Foundation (GB-3896).