Postnatal development and properties of N-methyl-d-aspartate (NMDA) receptors were studied with whole-cell and outside-out patch-clamp techniques in interneurons and fluorescence-labelled motoneurons in rat spinal cord slices. Both the absolute amplitude of NMDA-induced currents and currents normalized with respect to the motoneuron capacitance increased significantly at postnatal days 10-13 when compared to the responses evoked at postnatal days 2-3. The mean amplitude of the responses to kainate also increased in motoneurons of postnatal days 10-13. Single-channel currents induced by low concentrations of glutamate, exhibited four distinct amplitude levels corresponding to 19.2 +/- 2.4 pS, 38.4 +/- 3.5 pS, 56.3 +/- 2. 4 pS and 69.6 +/- 3.7 pS. In contrast, the conductance of single channels, recorded under identical conditions, in rat spinal cord interneurons was less, 15.3 +/- 3.2 pS, 29.9 +/- 5.4 pS, 46.7 +/- 4. 8 pS and 62.4 +/- 3.9 pS. The high (56/70 pS) conductance single-channel openings in motoneuron patches were sensitive to NMDA receptor inhibitors D-2-amino-5-phosphonovalerate, 7-chlorokynurenic acid and ifenprodil. Whole-cell NMDA-evoked currents were blocked in a voltage-dependent manner by extracellular Mg2+ with an apparent dissociation constant for Mg2+ binding at 0 mV of 1.8 +/- 0.5 mm. The conductance and relative distribution of NMDA receptor channel openings induced by 1 micrometer glutamate in patches isolated from the motoneurons were independent of age from postnatal day 4 to 14. The results suggest that the properties of NMDA receptor channels in motoneurons differ from those in spinal cord interneurons and cells transfected with NR1/NR2 subunits.