Current conduction in organic field-effect transistors (OFETs) has attracted much attention. The most intriguing issue is that for most organic semiconductors only one carrier type is observed. To examine why this is so, effort has been spent to study ambipolar conduction in OFETs under strong gate bias. For the rubrene OFETs analyzed in this work, there is evidence that p-channel conduction, which occurs at low positive gate bias, is closely associated with negative charge states present at the insulator-semiconductor interface. Because of the somewhat small insulator capacitance only a fraction of the applied gate voltage drops across the OFET channel making it very difficult to achieve n-channel conduction at low positive gate voltage. Furthermore, electron conductivity is usually low due to the smaller density of states in the LUMO. As computed, the negative charge states appear to have peak energy at 0.8 eV above the HOMO and it is tempting to associate them with "polaron" states found in the insulator.