The insulin receptor and its regulation by insulin was studied in U-937 monocytes, a human cell line with properties similar to those of normal peripheral blood monocytes. Treatment of this cell with insulin for 8-16 h produced an overall loss in the insulin receptor, i.e., a loss of receptors from the cell surface and internal pools. In contrast, short-term insulin treatment (15-30 min) caused a reduction in cell surface receptors but an increase in the internal receptors, as judged by pronase treatment at 4 degrees C to distinguish receptor location. After the removal of insulin and pronase, the internalized receptors were rapidly reinserted back into the cell surface after warming to 37 degrees C. Further studies showed an insulin-mediated increase in fluid-phase pinocytosis as measured by horseradish peroxidase (HRP) uptake. The amount of HRP accumulation and the time course for this stimulation were similar to those for receptor internalization. These features plus other results suggest that the insulin-stimulated internalization of insulin receptors may require an acceleration in the rate of pinocytic vesicle formation.