The aim of this work was to study the effects of protodioscin ingestion in two different flocks of sheep: a flock of 23 crossbreed Mato Grosso do Sul Native sheep raised on Brachiaria spp. pastures from birth (experienced flock) in the state of Mato Grosso do Sul; and another flock (naïve flock) of 18 crossbred Dorper × Santa Inês sheep raised in the state of Paraná in Paspalum notatum and Lolium multiflorum pastures. The two flocks grazed together in a Brachiaria brizantha pasture during a 140-day period in the rainy season. At the beginning of the experiment and every 14 days thereafter, blood samples were collected for determination of serum activities of gamma-glutamyl transferase (GGT) and aspartate aminotransferase (AST), and for determination of the icterus index. On the same days, samples of young, mature and old B. brizantha leaves were collected for protodioscin quantification. Naïve sheep were more susceptible to poisoning by B. brizantha than experienced sheep. Six sheep in the naïve flock were poisoned, and two of these died. Two sheep in the experienced flock were poisoned, and one of them died. The mean activities of serum GGT and AST were significantly higher in the naïve flock, also evidencing a higher susceptibility to the poison. These results suggest that flocks of sheep include animals with different degrees of resistance to Brachiaria spp. poisoning and that culling the susceptible animals may considerably increase of the resistance of the flock. The clinical signs and the lesions were similar to those previously reported. However, in sheep with black coats, the main clinical sign was weight loss without photosensitization-mediated dermatitis. One sheep from the experienced flock presented cirrhosis, with clinical signs of exercise intolerance. The protodioscin concentration (% DM) ranged from 0.87% to 2.58% (mean ± SD: 1.64 ± 0.58) in young leaves, 1.16%–2.53% (1.67 ± 0.44) in mature leaves and 0.98%–2.07% (1.52 ± 0.37) in old leaves. A negative relationship was found between saponin concentration and total cumulative precipitation.