Activity was recorded from mitral cells in newborn to six-day-old rat pups during odorous stimulation. Twenty-eight neurons were studied in pups with unopened nasal cavities which sampled stimuli during intermittent periods of inhalation. Forty-six neurons were studied in pups with opened nasal cavities which were stimulated by delivering odorants directly to the olfactory epithelia. We show that mitral cells are selectively excited by different odorants on the day pups are born; prior to the maturation of bulb interneurons, the responses of neonatal mitral cells are time-locked to the inhalation cycle; neonatal mitral cells preserve the temporal patterns of activity exhibited by receptor neurons during stimulation with different concentrations of odorants; and the response patterns of mitral cells differ qualitatively between newborn and adult rats. We conclude that receptor-to-mitral cell synapses are functional in newborn rat pups and that the activity of this afferent pathway is modulated by the pups' respiratory behavior. We argue that without interneurons, mitral cells repeat the temporal code exhibited by receptor neurons and do not produce the types of response patterns characteristic of neurons in the adult rat olfactory bulb.