To investigate the influence of daylength on the seasonal reproductive cycle of stallions, 21 stallions were assigned to one of three treatments: control, ambient (natural) photoperiod; S-L, 8 h light and 16 h dark (8:16) for 20 wk beginning July 16, 1982 then 16:8 from December 2, 1982 until March 5, 1984; S-S, 8:16 from July 16, 1982 until March 1984. Temperature was not controlled and was similar for all groups. Total scrotal width (TSW) was measured every 4 wk throughout the experiment. During 10 periods, semen was collected and evaluated every other day for 3 wk and sexual behavior was assessed. The S-L stallions exposed to 16 h light in December had twice as much sperm output in February than in November. Within the February collection period, the sperm output for S-L stallions was greater (P less than .05) than that for either control of S-S stallions. The stimulatory effect of the S-L photoperiod also resulted in larger (P less than .05) testes and decreased (P less than .05) time to ejaculation for S-L stallions in February as compared with either controls or S-S stallions. Despite continued exposure to a 16:8 photoperiod, TSW and sperm output for S-L stallions eventually declined; presumably a consequence of photorefractoriness. The S-S stallions had seasonal cycles coincident with those for control stallions. Based on a sine wave model for TSW and sperm output, stallions in all three groups displayed significant seasonal cycles.(ABSTRACT TRUNCATED AT 250 WORDS)