Continuous twenty-four hour EEG recordings were conducted on freely-moving DBA/2 and C57BL/6 inbred mice. No brief spindle episodes (BSEs: 6-7 cps, 1-5 sec duration, high amplitude spindle bursts) were seen in the waking EEG of C57BL/6 mice. BSEs were a conspicuous element of the EEG during active waking (AW) and quiet waking (QW) in DBA/2 mice. BSEs occurred at a 10X faster rate in QW than in AW and had a longer duration. Sleep spindle bursts resembling BSEs were seen in both C57BL/6 and DBA/2 mice, and occasionally were observed to follow a K-complex. Rostropontine, but not midpontine, brainstem transection released spindles in both strains. Pentobarbital produced spindles in both strains. The waveforms of the waves comprising BSEs, sleep spindles, transection-induced spindles and barbiturate spindles were quite similar, though differing in frequencies and amplitude. Genetic factors may be critical for the lack of BSEs during AW and QW in C57BL/6 mice and for the occurrence of BSEs during AW in DBA/2 mice. In contrast, most other rodents whow a third pattern: BSEs only during QW. Since C57BL/6 mice can generate spindles under some circumstances, the absence of spindles during waking reflects some alteration in the mechanisms that control the initiation of BSEs rather than a lack of the circuits required to generate a BSE. These mechanisms are distinct from those processes of arousal that produce the background EEG desynchronization of waking. Following both rostropontine and midpontine transection, the background EEG is desynchronized, yet after rostropontine, but not midpontine transection, BSEs occur freely, at a rate over 200 per hour.