1. Rainfall data were collated for years preceding historical outbreaks of Mastomys rats in East Africa in order to test the hypothesis that such outbreaks occur after long dry periods.
2. Rodent outbreaks were generally not preceded by long dry periods.
3. Population dynamics of Mastomys natalensis rats in Tanzania are significantly affected by the distribution of rainfall during the rainy season.
4. All previous rodent outbreaks in Tanzania were preceded by abundant rainfall early in the rainy season, i.e. towards the end of the year.
5. A flow chart is constructed to assess the likelihood of rodent outbreaks up to 10 months beforehand, utilizing rainfall data.