Abstract: A downhole adjustable bent sub is provided for use in a drilling process in which a drill bit is subjected to a drilling force. The bent sub has a tubular housing and a tubular mandrel. The tubular housing has a bend along its length to define an upper part and a lower part, the respective longitudinal axes of which intersect at an angle. The tubular mandrel also has a bend along its length to define a first part and a second part, the respective longitudinal axis of which intersect at an angle. The first part of the mandrel slidably engages the interior of the lower part of the tubular housing. a longitudinal stop prevents the mandrel from being slid out of the housing. Longitudinal biasing means urge the mandrel toward the longitudinal stop to a first position. A rotational stop prevents relative rotation between the mandrel and the housing when the mandrel is in the first position.