An in situ underground sample analyzing apparatus for use in a multilevel borehole monitoring system is disclosed. A casing assembly comprising a plurality of elongate tubular casings (24) separated by measurement port couplers (26) is coaxially alignable in a borehole (20). The measurement port couplers (26) include an inlet measurement port (70b) for collecting fluid from an underground measurement zone (32) and an outlet measurement port (70a) for releasing fluid into the measurement zone (32). An in situ sample analyzing probe (124) is orientable in the casing assembly. The in situ sample analyzing probe (124) includes inlet and outlet probe ports (148b and 148a) alignable and mateable with the inlet and outlet measurement ports (70b and 70a). The inlet and outlet measurement ports (70b and 70a) typically include valves. When the operation of the in situ sample analyzing probe (124) causes the valves to open, the interior of the in situ sample analyzing probe (124) is then in fluid communication with the exterior of the measurement port coupler (26). A circulating system located in the in situ sample analyzing probe circulates fluid collected through the inlet probe port (148b) of the in situ sample analyzing probe (124) and the inlet measurement port (70b). The collected fluid is analyzed by chemical analyzing apparatus in communication with the circulating system. After in situ analysis, the circulating system releases at least a portion of the fluid through the outlet probe port (148a) and the outlet measurement port (70a) into the measurement zone (32). Alternatively, collected fluid can be stored for transportation to the surface for offsite analysis.