The interaction between soluble (Actrapid HM) and Lente (Monotard HM) human insulin preparations was examined in normal subjects. Incremental plasma insulin levels were determined following the subcutaneous administration of 6 U of soluble insulin admixed with 0.14 ml soluble insulin diluting medium, or 0.14 ml Lente diluting medium and injected either immediately or 5 min after preparation. For comparison separate but simultaneous injections of soluble (6 U) and Lente insulin (14 U) were administered subcutaneously. All injection volumes were identical. The incremental insulin levels were significantly greater between 60 and 90 min (p less than 0.05-0.01) following the administration of soluble insulin admixed with its own medium compared to the soluble insulin/Lente medium admixtures injected immediately and 5 min after preparation. There was no difference, however, in the plasma insulin profiles between the two soluble insulin/Lente medium schedules. The separate simultaneous administration of soluble and Lente insulin resulted in significantly higher plasma insulin levels at 30 and 90 min (p less than 0.05) when compared to the admixture injected immediately after preparation and from 30-120 min (p less than 0.05-0.01) compared to the admixture injected after 5 min. The two soluble/Lente insulin admixtures achieved similar plasma insulin profiles. Therefore when soluble human insulin is admixed with Lente human insulin and administered by subcutaneous injection immediately after admixing there is a significant reduction in the plasma insulin levels during the first 90 min in contrast to when the two preparations are given simultaneously by separate injection. Delaying the injection of the admixture for 5 min results in significantly lower insulin levels up to 120 min. These differences are reflected in the hypoglycaemic responses observed.