BACKGROUND: Due to the incomplete microbial population of the gastrointestinal tract in infant animals, the occurrence of any kind of stress causes gastrointestinal microbial imbalance and gastrointestinal disorders in the animal. The use of probiotics in diets of experimental animals improves the production of volatile fatty acids in rumen as the main stimulants of ruminal papillae. In this case the beneficial microbial population promotes in the rumen and its products improve the health and animal performance. OBJECTIVES: The aim of this study was to investigate the effects of different levels of Protexin™ probiotics in milk replacer on digestive tract development and ruminal parameters of suckling Zel lambs. METHODS: To conduct this experiment, 24 male lambs were used at 10 days of age with mean weight (4.2 ± 0.53 kg) in 4 treatments and 6 replicates per each treatment as individual pens for 60 days. Treatments include control (without probiotic) and 3, 6 and 9 g (×109 cfu/g) of probiotic in milk replacer. In day 60 of experiments, pH values were measured by a portable digital device and ammonia nitrogen and number of protozoa in rumen fluid were measured in laboratory. Ruminal morphological results were studied after animal was slaughtered. RESULTS: The results of ruminal parameters showed that adding 9 g probiotic resulted in a significant increase in ammonia nitrogen and number of ruminal fluid protozoa compared to other levels (P<0.05). There were not significant differences in pH and VFA of ruminal fluid indicated no significant difference among treatments. In rumen morphology results, there was a significant difference between treatments in weight of empty whole digestive tract, rumen, omasum and abomasums weight and the volume of reticulum and abomasum (P<0.05) and this difference was significant between treatment of 9 g probiotic and other treatments. Also, the effect of experimental treatments was significant on length and width of the rumen papillae (P<0.05), so that level of 9 g probiotic was higher than other levels. CONCLUSIONS: The results showed that the addition of probiotic in milk replacer of experimental lambs significantly increased ruminal VFA and developed the ruminal morphological traits, so that in these traits, amount of 9 g probiotic had better performance than the other levels.