Holstein cows were fed fresh hay with a high sugar content and treated with 6% liquid ammonia as their main ration, for 3 weeks before and after calving. Their colostrum (test milk) was compared to normal colostrum to which 4 methyl-imidazole (4MI milk) had been added, and to normal colostrum lacking 4MI (control). Ten male Holstein calves (2 hours to 12 days of age) were used in this trial. Following the consumption of test milk, 4MI milk or normal colostrum, the symptoms of a central nervous system disorder were compared. Additional parameters monitored included heart function analyzed through electrocardiogram (ECG), auditory brainstem response (ABR), hematological and blood chemistry profiles. Two calves became febrile, hyperexcitable, and displayed an abnormal circling behavior within 2 days after the consumption of test milk. ABR decreased and disappeared intermittently. Four out of 6 calves given 4MI milk developed similar signs and showed a similar disappearance of ABR, while 2 older calves (6 and 12 days) did not develop fever or did not exhibit circling or abnormal ABR. Nearly all the experimental calves showed an elevation of blood pyruvate and lactate concentrations coinciding with the onset of illness. No remarkable changes in other blood components were detected. The autopsy did not reveal any pathological changes. These findings suggest that the age of the calf is related to the toxicity of a compound and in this experiment therefore affected the comparison of clinical signs induced by either 4MI milk or test milk.