Under moderate latitudes all breeds of rams undergo seasonal variations in testicular weight with a maximum during summer under decreasing daylength ([1]-[4]). Similarly, in rams submitted to a 6-month artificial light regime [5] or to an alternation of long (16L:8D) and short (8L:16D) days [6] an increase in testicular weight occurred following a decrease in daylength and vice versa. However this effect is transitory, a phenomenon which can be referred as photorefractoriness. In the present study the influence of the period of the light cycle on variation in testicular weight in the ram was investigated. 4 groups of 6 adults Ile-de-France rams were submitted to artificial light cycles where the daylength varied between 8-16 hrs. and the period (T) was 6, 4, 3, or 2 months respectively (Groups T6, T4, T3, and T2). Testicular volume was measured fortnightly using an orchidometer, Variations in testicular volume were submitted to harmonic regression analysis following the model y(t)=mu + a sin(2(pi t/tau) + phi). Cyclic changes in testicular volume were seen with each light cycle, at least in groups T6, T4, and T3 (Fig.). Analysis (Table) showed that: (1) the coefficient of determination R2 was high in the groups (2) mean testicular volume has increased from 258 to 294 cm3 when the period of the light cycle decreased from 6 to 2 months; (3) conversely, the amplitude decreased from 66.5 to 26.5 cm3 as the period decreased; (4) maximal testicular volumes (mean plus amplitude) were similar in all groups (range: T4, 312,5-T6,324 cm3) while minima (mean less amplitude) differed significantly (P<0.000,1) between groups (range: T6 and T4 about 190, T2 267.5 cm3) and (50 th computed periods of testicular volumes cycles were almost identical to the imposed light cycles.(ABSTRACT TRUNCATED AT 250 WORDS)