Outbreaks of an often fatal disease of cattle grazing Kikuyu grass (Pennisetum clandestinum) occur sporadically during late summer and autumn in northern areas of New Zealand. The disease is commonly known among veterinarians and farmers as “Kikuyu poisoning”. The writers will use this name but consider that factors other than Kikuyu grass per se may be important in the aetiblogy of the condition. The first reports in the veterinary literature of Kikuyu poisoning were those of Cordes et al (1969) and Busch et al (1969). Similarities between, the disease and two experimental mycotoxicoses have been described (Martinovich et al 1972); however, its cause is at present unproven. Sheep may be affected (Martinovich and Smith, 1972) but appear less susceptible than cattle. In the present series of two papers existing knowledge of Kikuyu poisoning is reviewed and features of diagnostic significance are discussed. This paper deals with clinical and pathological changes in affected animals. The information presented was obtained during investigations of 32 outbreaks of Kikuyu poisoning on separate farms in Northland during the period 1970 to 1972 (The Northland region is here regarded as that partt of the North Island lying north of Auckland city.) The second paper of the series will cover epizootiological aspects of the disease.