I figured it out. It's actually a DropDown object I'm interested in...
protected static Excel.Application app;
protected static Excel.Range range;
protected static Excel.Workbook wb;
protected static Excel.Sheets sheets;
protected static Excel.Worksheet sheet;
protected static object oMissing = Missing.Value;
range = sheet.get_Range("N1", "N1");
Excel.DropDowns xlDropDowns;
Excel.DropDown xlDropDown;
xlDropDowns = ((Excel.DropDowns)(sheet.DropDowns(oMissing)));
xlDropDown = xlDropDowns.Add((double)range.Left,
(double)range.Top, (double)range.Width, (double)range.Height, true);
//Add items into drop down list
for (int i = 0; i < items.Length; i++)
{
xlDropDown.AddItem(items, i + 1);
}