This study introduces novel aspects of inward rectification in neonatal rat spinal motoneurons (MNs) and its modulation by serotonin (5-HT). Whole cell tight-seal recordings were made from MNs in an isolated lumbar spinal cord preparation from rats 1-2 days of age. In voltage clamp, hyperpolarizing step commands were generated from holding potentials of -50 to -40 mV. Discordant with previous reports involving slice preparations, fast inward rectification was commonly expressed and in 44% of the MNs co-existed with a slow inward rectification related to activation of I_h. The fast inward rectification is likely caused by an I_Kir. Thus it appeared around E_K and was sensitive to low concentrations (100-300 μM) of Ba²⁺ but not to ZD 7288, which blocked I_h. Both I_Kir and I_h were inhibited by Cs²⁺ (0.3-1.5 mM). Extracellular addition of 5-HT (10 μM) reduced the instantaneous conductance, most strongly at membrane potentials above E_K. Low [Ba²⁺] prevented the 5-HT-induced instantaneous conductance reduction below, but not that above, E_K. This suggests that 5-HT inhibits I_Kir, but also other instantaneous conductances. The biophysical parameters of I_h were evaluated before and under 5-HT. The maximal I_h conductance, G_max, was 12 nS, much higher than observed in slice preparations. G_max was unaffected by 5-HT. In contrast, 5-HT caused a 7-mV depolarizing shift in the activation curve of I_h. Double-exponential fits were generally needed to describe I_h activation. The fast and slow time constants obtained by these fits differed by an order of magnitude. Both time constants were accelerated by 5-HT, the slow time constant to the largest extent. We conclude that spinal neonatal MNs possess multiple forms of inward rectification. I_h may be carried by two spatially segregated channel populations, which differ in kinetics and sensitivity to 5-HT. 5-HT increases MN excitability in several ways, including inhibition of a barium-insensitive leak conductance, inhibition of I_Kir, and enhancement of I_h. The quantitative characterization of these effects should be useful for further studies seeking to understand how neuromodulation prepares vertebrate MNs for concerted behaviors such as locomotor activity.