Aims/hypothesis. The aim of this study was to examine factors involved in chylomicron production in the streptozotocin diabetic rat, our hypothesis being that the synthesis of the chylomicron is abnormal in diabetes. Methods. Diabetic rats (n = 20) were paired with control rats (n = 20). Cholesterol emulsion was given by gavage and the lymph duct was cannulated. Lymph was collected for 4 h. Chylomicrons were prepared from the lymph by ultracentrifugation. Lymph apolipoprotein B48 was isolated by gradient gel electrophoresis and quantified by densitometric scanning. Intestinal microsomal triglycerol transfer protein mRNA was measured by solution hybridisation nuclease protection, using a rat specific [³²P]-labelled cRNA probe. Results. Serum triglyceride and cholesterol were greatly increased in diabetic compared with control animals (258 ± 77 and 8.9 ± 6.4 mg/ml vs 1.04 ± 0.37 and 0.54 ± 0.03 mg/ml, p < 0.0001). Lymph chylomicron triglyceride and cholesterol were also higher in diabetic rats (29.4 ± 27.3 and 0.28 ± 0.3 mg/h vs 16.8 ± 10.6 and 0.18 ± 0.09 mg/h, p < 0.05). Lymph chylomicron apo B48 was similar in the two groups. Intestinal microsomal triglycerol transfer protein mRNA was higher in the diabetic rats (12.6 ± 3.2 vs 3.8 ± 3.0 amol/μg RNA, p < 0.0001) and there was a positive correlation between lymph triglyceride and microsomal triglycerol transfer protein mRNA in the whole group (r = 0.65, p < 0.01). Conclusion/interpretation. The study shows that microsomal triglycerol transfer protein mRNA is raised in diabetes without an increase in apolipoprotein B48 in the lymph suggesting that microsomal triglycerol transfer protein regulates chylomicron triglyceride content but not particle number. [Diabetologia (1999) 42: 944–948]