SummaryThis study was conducted from June 1968 to July 1969 and from August 1970 to December 1972 with information available for the intervening period as well as for the first half of 1968, giving records for approximately 5 years.The Nairobi Park lion population has averaged thirty animals with a high percentage (about 60%) of juveniles and a preponderance of females among the adults.Matings occurred all through the year but births showed a distinct peak during December and January. The correlation between number of kills one month and births 3 months later proved statistically significant, suggesting that the level of nutrition may influence fertility.Gestation period is about 110 days; litters, when first seen, averaged three cubs, sex ratio was 1:1. Cubs ioined their pride when 6–8 weeks old, but only if no cubs much older than themselves were present; reasons for this are suggested.Young males left their pride around the age of 2 years and so did most young females, although the latter occasionally stay on with their pride.Youngest age to conceive was about 2| years, intervals between litters averaged 2 years. The hypothesis is put forward that lionesses usually refuse to raise single cubs or those that remain sole survivors at less than 3 months of age. Some evidence to support this thesis is submitted.The reproductive performance of the Nairobi Park lions is compared with those in the Serengeti (Schaller, 1972). The Nairobi population's higher performance despite a higher density there suggests that numbers are still within the optimum. However, this balance is dependent upon a large surplus being able to disperse into the area adjo'ning the Park