Selfheating effects in silicon resistors operated at cryogenic ambient temperatures (T(a)) are discussed. A novel method to determine directly the local temperature of the silicon device when operating at cryogenic temperatures is presented. By using this novel method, experimental results are presented in which selfheating effects in both types of silicon resistors, p-type in p-substrate and n-type in n-well, operated at liquid helium temperature, are examined. The value of the measured time (tau(th)) the silicon takes to cool down to the ambient temperature T(a) after being heated up by a current flow through the silicon indicates that selfheating has a strong impact on the hysteresis effect observed in the I-V characteristics at 4.2 K. It is also demonstrated that the critical power dissipation at which selfheating starts to have a significant effect on the I-V characteristics of silicon resistors, is much larger for resistors fabricated in the substrate than those fabricated in the well. The observed selfheating effect is explained in terms of the thermal properties of silicon.