The aim of this work was to determine the effect of the season of the year (photoperiod) on scrotal circumference (SC), testicular weight (TW), testicular volume (TV), serving capacity test (SCT), plasma testosterone concentration and semen quality traits in Highlander rams. The study was carried out in the humid pampas of Argentina (130 m above sea level) with six adult rams (5-6 years old). Monthly determinations of body weight, body condition score, SC, TW [TW = 0.5533 x (L) x (W)2], TV [TV=4/3 x (π) x (L/2) x (W/2)2], being L=long and W=width of the testicle, and plasma testosterone. The SCT was performed twice per season and the semen evaluation twice per week during the central month of each season. The analysis was done using linear mixed models for repeated measures (R Core Team). In summer (March) the highest values of SC (mean ± SE; p<0.05) (34.2 ± 0.7 cm), TW (506.7 ± 2...