<b>Background and Objective:</b> Photoperiod can regulate reproductive physiological processes in mammals, in which improvements in testosterone concentration, testicular volume and seminal quality have been reported. The aim was to evaluate the influence of photoperiod treatments on guinea pigs' spermatic parameters. <b>Materials and Methods:</b> Thirty guinea pigs, between males and females, were distributed in two rooms with the photoperiodic treatment of 10 hrs light and 14 hrs dark (PT<sub>1</sub> with artificial photoperiod and PT<sub>2</sub> photoperiod with sunlight by opening windows from 08:00-18:00) and one without any direct light stimulus (PT<sub>0</sub>) for 78 days. The temperature and humidity were recorded and the TH index was calculated for each room. The sperms were recovered in Tris base medium from the epididymis of 16 males to determine sperm concentration, motility, kinetic parameters, vitality, HOST, acrosomal integrity and DNA fragmentation. <b>Results:</b> Sperm values in PT<sub>1</sub> and PT<sub>0</sub> were similar but PT<sub>2</sub> obtained values lower in sperm concentration, non-progressive motility, total motility, VCL, ALH, vitality, HOST+, acrosomal integrity, sperm with non-fragmented DNA and no pregnancies were reported (0/5). A 100% pregnancy was observed in PT<sub>0</sub> (4/4) and 50% in PT<sub>1</sub> (2/4). However, precocity was evidenced in PT<sub>1</sub> compared to PT<sub>0</sub>. PT<sub>2</sub> recorded higher peaks in temperature (33.8°C, THI 81, considered as thermal stress) compared to PT<sub>0</sub> (32.65°C, THI 81.8) and PT<sub>1</sub> (32.75°C, THI 81.6). <b>Conclusion:</b> An artificial photoperiod can improve sperm characteristics and reproductive precociousness of guinea pigs, unlike the photoperiod with sunlight, which generated low spermiogram values and absence of pregnancy due to thermal stress.
Keywords: Cavia porcellus L.; DNA fragmentation; HOST; TH index; acrosome; kinetic parameters; photoperiod; spermiogram; sunlight; thermal stress.