Blood samples were collected from 2 female manatees - one born in captivity 3 May 1975 (5 years of age) and the other acquired as an infant less than 1 month old 25 December 1974 (6 years of age).  Six samples were collected from 3 July 1980 to 21 January 1981 at approximately monthly intervals.  Serum levels of estrogen and progesterone were determined by radioimmunoassay.  Results indicate that neither animal had achieved puberty and regular reproductive cycle activity.  The adult male which was housed with the juvenile females shows little interest in them although he was sexually active with the 2 adult females (10 years of age) also present in the tank.  It has been reported that the manatee reaches sexual maturity at 8 (±2) years and attain an age of more than 30 years in captivity.  The calving interval has been 5 to 7 years in the captive group we are studying. Based on puberty at 6 years, a calving interval of 3 years and reproductive activity until 30 years of age, the reproductive efficiency of the manatee of 0.27/year/female. The average loss per year for the Florida manatee over the last 3 years is approximately 90 animals.  If the data for captive animals can be applied to wild populations it would require a minimum of 334 females to sustain the population in Florida.