With the aim of automatically generating head motions from speech, analyses are conducted for verifying the relations between head motions and linguistic and paralinguistic information carried by speech. Analyses are conducted on motion captured data during natural dialogue. Analysis results showed that nods frequently occur during speech utterances, not only for expressing dialog acts such as agreement and affirmation, but also as indicative of syntactic or semantic units, appearing at the last syllable of the phrases, in strong phrase boundaries. The paper also analyzes the dependence on linguistic, prosodic and voice quality information of other head motions, like shakes and tilts, and discuss about the potentiality for their use in automatic generation of head motions.