Two experiments were designed to evaluate the effect of silymarin on stored spermatozoa using four rams. In experiment 1, silymarin was evaluated as a supplement for Tris–glucose extender. Semen samples (n = 20) were diluted with extender containing 0, 50, 100, 150 and 200 μg/ml silymarin and incubated at 5°C for 72 h. Membrane integrity, acrosome integrity, sperm viability and motility were evaluated at 72 h. Concentration of malondialdehyde (MDA) was determined after 48 h. Membrane integrity was higher in 100 μg/ml silymarin (65.2%) than control group (43.2%, p < 0.05). Acrosome integrity was highest in 100 μg/ml silymarin (71.3%, p < 0.05). Progressive motility was higher in 100 (58.5%), 150 (60.62%) and 200 μg/ml silymarin (54.7%) than control group (30.7%, p < 0.05). The highest MDA concentration was observed in control group (400 mm/10 × 106 sperm; p < 0.05). The goal of experiment 2 was to determine the interaction between silymarin and caproic acid on ram stored sperm. Ejaculates (n = 20) were diluted by Tris–glucose extender, added 0 (S−) or 100 μg/ml (S+) silymarin and 0 (C−) or 0.3125% (C+) caproic acid, and thereafter, aliquots were incubated at 5°C for 72 h. Membrane integrity was lower in C−S− (57.6%) than C−S+ (73.2%), C+S− (80.2%) and C+S+ (72.1%, p > 0.05). The highest sperm viability and acrosome integrity were observed in C+S- (82.4 and 80.1%, respectively; p < 0.05). There was no difference between C-S+ and C+S+ on sperm viability and membrane integrity, progressive motility and MDA concentration (p > 0.05). Therefore, the supplementation of extender with silymarin and caproic acid improved sperm quality and caproic acid was superior to caproic acid plus silymarin.