Six normal subjects received subcutaneous human, porcine, and bovine ultralente insulin (0.30 U/kg) and diluent (control) in randomized order. Plasma glucose, C-peptide, and insulin were measured for 32 h after injection. From 10 h onward human ultralente produced significantly lower plasma glucose levels (p less than 0.05-0.01) compared to bovine ultralente. Porcine ultralente produced an intermediate hypoglycaemic response up to 16 h and was similar to the bovine insulin from 24-32 h. Estimated exogenous insulin concentration was higher (p less than 0.05-0.001) following human ultralente compared to bovine ultralente between 2 and 22 h after injection. Up to 24 h the porcine preparation led to intermediate insulin levels, but becoming identical to bovine ultralente from 28-32 h. Peak mean exogenous insulin values for human, porcine, and bovine ultralente were 0.054, 0.044, and 0.023 nmol/l at 14, 16, and 18 h, respectively, reaching 0.022, 0.013, and 0.013 nmol/l at 32 h. The different pharmacokinetic behaviour of human and bovine ultralente insulin must be considered when initiating treatment with human ultralente or transferring patients from bovine to human ultralente.