In accordance with illustrative embodiments of the present invention, a measuring-while-drilling system includes a sensor sub positioned at the lower end of a downhole motor assembly so that the sub is located near the drill bit. The sub houses instrumentalities that measure various downhole parameters such as inclination of the borehole, the natural gamma ray emission of the formations, the electrical resistivity of the formations, and a number of mechanical drilling performance parameters. Sonic or electromagnetic telemetry signals representing these measurements are transmitted uphole to a receiver associated with a convential MWD tool located above the motor, and telemetered by this tool to the surface substantially in real time. The system has particular application to accurate control over the drilling of extended reach and horizontally drilled wells.