The episodic nature of LH [luteinizing hormone] secretion in sheep exposed to different photoperiods was investigated. The switch from long to short daylengths induced testicular growth after about 3 wk, which continued until daylengths were extended again. Plasma LH levels also changed in relation to the photoperiod. The 1st greatly elevated LH values occurred 17 days after the switch from long to short daylengths, slightly in advance of the initiation of testicular growth. Plasma values remained increased for much of the remaining short-day period, although there was great variability, and levels declined in later weeks. The return to long day-lengths further decreased LH values. Hourly bleeds confirmed these changes and explained some of the variability. During the long-day photoperiods LH levels were uniformly low throughout the 24 h, and peaks were seldom evident. After the change to short daylengths, the frequency and magnitude of the LH peaks increased with an increase in the minimum values. At the end of the short-day period the frequency of peaks was still at a maximum, but the amplitude had declined. The return to long daylengths decreased all parameters of the LH concentrations with a loss of major episodic peaks within 5 wk. The episodic release of LH may be profoundly altered by changes in the photoperiod. A change from long to short daylengths appears to switch on the hypothalamic control of LH release, but it takes many weeks to attain maximal secretion; testicular growth takes longer. These slow responses reflect the long time period necessary for the complete development of the pituitary-testis system after activation.