The plasma profiles of growth hormone (GH) in adult male and female Atlantic salmon were determined in relation to manipulation of the photoperiod and to the development and timing of sexual maturation. Fish were exposed to natural light (NL) or NL + 24L:0D additional light over the netpens from January (ALJ) or March (ALM) to July. Thereafter, these groups were brought indoors, subdivided, and subjected to simulated natural photoperiod (SNP), continuous light (24L), or short day (8L). Assay of salmon GH by RIA in monthly plasma samples revealed that GH levels were generally < 1 ng ml-1 during January to June and were only slightly affected by additional light in January or March. ALJ-24L treatment, and to a lesser extent, ALM-24L treatment, was effective in preventing sexual maturation, and GH levels of immature fish continued to be < or = 1.5 ng ml-1. On the other hand, in sexually maturing fish, GH levels increased to 2-5 ng ml-1 months prior to ovulation. Short-day photoperiod (8L) from July advanced ovulation and spermiation, whereas continuous light from July delayed these processes. The timing of the increase of GH levels was shifted in a parallel manner, indicating a functional relationship between plasma GH levels and the process of sexual maturation.