Smolting juvenile coho salmon were sampled to determine (1) whether a correlation between hypo-osmoregulatory ability and visual pigment composition existed and (2) whether the hormone 3,5,3'-tri-iodothyronine (T3) was playing a role in the visual pigment conversion process. Plasma sodium levels of seawater-challenged fish (30 ) indicated that there was a 5 week period of optimal ability to excrete excess plasma sodium ions (hypo-osmoregulation) in the late spring/early summer that represented the 'window of opportunity' for the entry or introduction to sea water of the salmon. Early in the smoltification process, the vitamin-A2-based visual pigment porphyropsin increased its dominance in the retinae, and radioimmunoassay of plasma indicated that T3 levels were at a maximum prior to this increase in porphyropsin. As the parr­smolt transformation continued, there was a steady decrease in the relative amounts of porphyropsin, indicating that the retinae were favouring the acquisition of rhodopsin. Rhodopsin dominance virtually coincided with the period of best hypo-osmoregulatory ability. Subsequently, the salmon showed a loss of hypo-osmoregulatory ability and concomitant increases in the amount of porphyropsin in the retina were observed. The relationship between the visual pigment shift and the smoltification process is discussed in terms of preparation for migration and thyroid hormone involvement, and the use of retinal visual pigment composition as an index of smolt status is proposed.