Captive breeding management through chemical contraception has had limited application in marine mammals. Aquarium facilities are increasingly faced with the challenging issues of inhibition of undesirable in-breeding, maintenance of genetic diversity, and management of populations. This study involves a 19 year old female killer who has experienced 3 unsuccessful attempts at calf-rearing. A review of the future of this species at the Vancouver Aquarium prompted an investigation into the logistics of temporary inhibition of estrus. Various fertility management protocols were considered on the basis of safety, efficacy, logistics of administration, and ease of monitoring. A synthetic progestin Altrenogest (Regu-mate) was chosen, and was administered daily starting 20 days postpartum. The equine dosage of 0.044 mg/kg Altrenogest was administered in her initial daily feed of herring. Monitoring of drug efficacy was accomplished by measuring pregnanediol glucuronide (PdG) and estrone conjugates (EC) in daily urine samples, and progesterone and B-17 estradiol in twice monthly serum samples. Ovarian activity was minimal throughout the 375 days of treatment. Pregnanediol glucuronide (PdG) levels ranged from 0.0 to 7.32 ng/mg creatinine (mean of 0.961), while estrone conjugates (EC) levels ranged from 0.0 to 2.64 ng/mg creatinine (mean of 0.253). No classical estrus activity was observed. However, from days 140 to 165 elevated levels of EC were measured along with aberrant elevations of PdG. A final analysis of this particular aberration will be available with the results from the radioimmunoassay of urinary luteinizing hormone. Sexual activity between the male and female has not diminished. Clinical and hematological parameters remain normal. Interim results suggest that the chemical contraceptive Altrenogest (Regu-mate) can provide reliable inhibition of estrus in the killer whale. Ongoing studies are designed to investigate regulation of the timing of estrus, long term cyclical contraceptive therapy, and measurement of serum altrenogest levels in killer whales.