The pharmacokinetics of five pre-mixed insulin preparations in the ratio of soluble to NPH insulin of 10:90, 20:80, 30:70, 40:60 and 50:50, were examined in a two part study in fasting healthy subjects. Each received by bolus subcutaneous injection into the anterior abdominal wall, on separate occasions one to two weeks apart, 20U of each of three pre-mixed insulin preparations in random order. In Part 1, nine subjects received Penmix 10:90, Penmix 20:80 and Penmix 30:70 and were observed over a period of 24 hours. In Part 2, eight subjects received Penmix 30:70, Penmix 40:60 and Penmix 50:50 and were observed over an 8 hr post-injection period. Three subjects were common to both parts of the study. Plasma glucose, C-peptide and insulin levels were measured frequently throughout both study periods. Increasing soluble insulin content in the pre-mixtures was reflected in increasing peak plasma insulin concentrations and a greater hypoglycaemic response. There were highly significant differences between the five premixtures and preparations in the 0-4 and 0-8 hours area under the curve (AUC) values for plasma glucose, C-peptide and immunoreactive insulin concentration (p < 0.01). Whereas a gradual difference between the premixtures was seen no two adjacent ones were significantly different, however an overall highly significant difference between the five preparations tested was observed.