After intracerebral inoculation of mice with a 10 per cent suspension (approximately 2000 LD50) of the Lansing strain of poliomyelitis virus, the infectivity titer in the brain decreased for approximately 6 hours. It then rose rapidly for 12 to 18 hours to reach titers of over 10–4. The rise in titer in the spinal cord closely paralleled that in the brain for 18 hours, after which the titer surpassed that in the brain by as much as one log. The infectivity titers in the central nervous system of unparalyzed mice remained between 10–3.5 and 10–4.2 for at least 7 days. With the onset of paralysis it was found that the titer was consistently and significantly higher in the spinal cords of paralyzed mice than in their brains or in the brains or cords of unparalyzed mice.

After inoculation of 1 per cent virus suspension the increase in titer occurred about 9 hours later than after the inoculation of 10 per cent virus suspension, and the onset of clinical signs of illness was also delayed. Once the titers began to rise, the rate was the same after the inoculation of either concentration of virus, and the maximal levels reached were the same. With both concentrations of virus, maximal infectivity titers in non-paralyzed mice were reached about 9 hours before the onset of signs of poliomyelitis.

The significance of these findings is discussed.

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