The objective of our study was to resolve two issues pertaining to motoneuron (MN) spike frequency adaptation (SFA): 1) to develop an index of SFA that is sensitive to a wide range in adaptation patterns and would correlate well with MN excitability and 2) to determine if SFA pattern is stimulus current-dependent. Sprague-Dawley rats (250 - 350 g) were anesthetized (ketamine/xylazine) prior to recording electrophysiological properties from sciatic nerve MNs located in the lumbar spinal cord. SFA was measured using 30-s square wave current injections at 1.5, 3.0, and 5.0 nA above estimated rhythmic firing threshold (R_th). Discharges per second were significantly (p < 0.001) higher for 5 nA versus the 1.5 and 3 nA currents >R_th in the first 2 s. SFA was quantified by using ratios of the final to initial number of discharges using 1-s, 2-s, and 5-s bins. The best index of SFA was the percent decline in the number of spikes fired in the 5^th 5-s bin relative to the first 5-s bin (1-(bin5/bin1)). Using this index, SFA significantly correlated with several measures of MN excitability, including estimated persistent inward current (PIC) amplitude (r=-0.76), rheobase current (r=0.71), and tended to correlate with input resistance (r=-0.43) and frequency-current (f-I) slope (r=-0.57). This index also showed the widest range of SFA among MNs. In conclusion, a SFA pattern can be ascertained for each MN and becomes more pronounced as MN excitability decreases. Finally, for the first time we report evidence of a relationship between PIC and SFA.