Abstract The physiology of the developmental relative hypoglycemia of ruminants has been related to forestomach development or the absorption of the end products of forestomach fermentation. However, few data are available to substantiate whether a cause-and-effect relationship exists. Calves were fed all of their solid food by abomasal fistula. Their forestomach developmental changes, and blood reducing sugar and blood organic acid levels, were compared with those of two oral-fed groups. At slaughter (115 days) only a few particles of solid food were found in the reticulo-rumens of the abomasal-fed calves. Significantly, essentially no rumen papillary growth had occurred in these calves, whereas the oral groups had marked papillary development typical for their respective types of feeding programs. Blood reducing sugar levels decreased and blood organic acid levels increased with age (both P<.01), but no significant differences occurred between groups. These data indicate that the ontogenical relative hypoglycemia of the calf is not caused by forestomach development, absorption of end products of forestomach fermentation, or digestion in the lower tract of microorganisms produced in the forestomach.