The relationship between high intake of rapidly fermentable carbohydrates and the subsequent response on plasma glucose, insulin sensitivity and clinical signs of laminitis in dairy heifers was studied. Ten dairy heifers with a mean body weight of 275 kg (SD 13.8 kg), were parallelly subjected (n=5/treatment group) to either a total mixed ration (TMR) low (LC, 130 g DM/kg) or high (HC, 480 g DM/kg) in cassava (starch). Results showed that mean DM intake, rectal temperature, heart rate, respiration rate and rumen contractions were not affected by the dietary treatments. All heifers fed the HC-TMR had diarrhea while all the heifers fed the LC-TMR had a pasty, soft consistency of feces. Three of the 5 heifers fed the HC-TMR displayed clinical signs of acute laminitis compared to none in the LC-TMR group. Mean postprandial rumen pH values of heifers fed the HC-TMR, but not the LC-TMR, were lower than 6.0 at all the time points measured. The reduction in rumen pH was associated with 49% greater concentration of total VFA and a concomitant shift from acetic to propionic acid and increased rumen lactic acid concentrations. Moreover, the HC-TMR caused greater plasma insulin levels and insulin resistance. The three laminitis animals had 1.9 greater basal plasma insulin values and a 46% lower insulin sensitivity values, respectively compared to their non-laminitis counterparts fed the HC-TMR. Insulin sensitivity was shown to be associated with clinical signs of laminitis in heifers only when plasma insulin concentrations were greater than ~ 700 pmol/L.