Recordings were made from neurons in the dorsal horn (DH), and from dorsal and ventral roots (DRs and VRs) of isolated spinal cords of infant mice. Raising calcium concentration ([Ca2+]) in the organ bath from 1.2 to 2.4 mmol/l resulted in a slight hyperpolarization, elevation of threshold current (rheobase), and augmentation of excitatory postsynaptic potentials (EPSPs). In many cells EPSPs acquired a much prolonged late phase. Orthodromic stimulation evoked in some DH neurons an action potential that had the same threshold as, and coincided in time with, the 'dorsal horn response' (DHR) recorded from DR. In spinal cords bathed in elevated [Ca2+], DR recordings showed irregularly recurring spontaneous waves, and DH neurons generated spontaneous EPSPs, often with spikes. Some neurons fired irregularly timed spontaneous action potentials that did not appear triggered by EPSPs. In less than 50% of the neurons the spontaneous EPSPs coincided in time with the spontaneous DR waves. The action potentials that appeared without EPSP were fired independently from DR activity. These observations confirm that elevation of interstitial free calcium concentration results in strong enhancement of excitatory transmission, especially of an EPSP of much extended duration. Virtually all neurons showed increased spontaneous activity in high [Ca2+], but only a minority appeared recruited into the synchronized discharges that are detectable as spontaneous waves in DR and VR recordings.