Absorption kinetics of regular and isophane (NPH) insulins were evaluated in seven normal fasted dogs by measuring serial serum concentrations of insulin and glucose following the subcutaneous administration of regular and NPH insulins. These results were compared to serum insulin values determined after injecting similar doses of regular insulin intravenously. Regular insulin was better absorbed than NPH insulin (mean bioavailability index 64.6% vs. 41.1%, P less than .01) resulting in a significantly greater maximal increase in mean circulating insulin concentrations above baseline values (362.2 microU/ml vs. 147.8 microU/ml, P less than .05). The time interval between insulin injection and return of serum insulin values to basal concentrations was also significantly shorter for regular than for NPH insulin (4.9 hr vs. 8.6 hr, P less than .05). However, there were no significant differences between regular and NPH insulins in time to reach peak serum insulin concentrations, maximal reduction in serum glucose concentrations, or time of lowest circulating glucose levels. The results of this study support previously accepted values for time-action characteristics of regular insulin, but suggest that NPH insulin may have an earlier peak and shorter duration of action than has previously been proposed in the dog.