Fescue foot, summer syndrome, reproductive problems, and ryegrass staggers are all diseases of livestock related to endophyte toxins in pasture grasses. Range finding experiments and case studies of fescue foot relative to ergovaline toxin found in endophyte infected tall fescue and lolitrem B present in endophyte infected perennial ryegrass were conducted. Within 42 d of initiating a feeding trial with chopped tall fescue straw containing 825 ppb ergovaline and at environmental temperatures of 15.9 C clinical signs of fescue foot were seen in cattle. Sheep on tall fescue pastures in November consuming feed with 540 ppb ergovaline and at environmental temperatures of 7.8 C developed fescue foot in 21 d while sheep on the adjacent field in the previous 2 mo with environmental temperatures of 16.6 C and 12.8 C and 458 ppb ergovaline in the pasture grasses did not. In a field outbreak of fescue foot affecting 42/425 feeder lambs in November, the ergovaline of sample pasture grasses had a mean concentration of 813 ppb. Perennial ryegrass staggers was seen in 42/237 feeder lambs when mean lolitrem B in the sampled grass was 2,135 ppb. Overgrazing both tall fescue and ryegrass fields increased probability of clinical disease since the highest levels of toxin were found in the crowns and basal leaf sheaths of tall fescue and perennial ryegrass respectively. Based on these findings, ergovaline dietary levels of 400 to 750 ppb to cattle and 500 to 800 ppb to sheep and lolitrem B levels of 1,800 to 2,000 ppb in feed for both species are approximated threshold values for disease. Cold environmental temperatures are equally important to toxin concentrations in precipitating fescue foot disease.