Reproductive fin whales (Balaenoptera physalus) (448 females and 278 males), classified by ana-tomical/histological methods, were studied for serum sex hormones. Of the 207 females classi-fied as pregnant by anatomical methods, 95% had progesterone (P) levels higher than 9.0 nmol/l with a near symmetrical distribution of log10 P-levels around the mean of 1.55 (35.5 nmol/l geometric mean). More than half of the sexually immature females (n = 157) had P-levels £ 0.1 nmol/l. Nonpregnant mature cows were older on average than pregnant cows, suggesting the begin-ning of reproductive senescence in females before the age of 30 y. The mean serum testosterone (T) of mature males (3.1 nmol/l) was significantly higher than that of immature males (1.0 nmol/l). In mature males, T-levels were positively asso-ciated with testicular size, as well as time (day-count) during the summer whaling season. For fin whales, serum P-and T-levels agreed closely with anatomical studies of reproduction and may be decisive when anatomical indexes fail. Furthermore, the serum T-level appears to be an excellent index for monitoring the latter part of the annual male reproductive cycle.
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