We have examined the interaction of photoperiod and temperature in regulating the parr-smolt transformation and its endocrine control. Atlantic salmon juveniles were reared at a constant temperature of 10??C or ambient temperature (2??C from January to April followed by seasonal increase) under simulated natural day length. At 10??C, an increase in day length [16 h of light and 8 h of darkness (LD 16:8)] in February accelerated increases in gill Na+K+-ATPase activity, whereas fish at ambient temperature did not respond to increased day length. Increases in gill Na+K+-ATPase activity under both photoperiods occurred later at ambient temperature than at 10??C. Plasma growth hormone (GH), insulin-like growth factor, and thyroxine increased within 7 days of increased day length at 10??C and remained elevated for 5-9 wk; the same photoperiod treatment at 2??C resulted in much smaller increases of shorter duration. Plasma cortisol increased transiently 3 and 5 wk after LD 16:8 at 10??C and ambient temperature, respectively. Plasma thyroxine was consistently higher at ambient temperature than at 10??C. Plasma triiodothyronine was initially higher at 10??C than at ambient temperature, and there was no response to LD 16:8 under either temperature regimen. There was a strong correlation between gill Na+K+-ATPase activity and plasma GH; correlations were weaker with other hormones. The results provide evidence that low temperature limits the physiological response to increased day length and that GH, insulin-like growth factor I, cortisol, and thyroid hormones mediate the environmental control of the parr-smolt transformation.