Abstract
1. 1. Seasonal variation in the plasma concentration of lutropin (LH), follitropin (FSH), prolactin (PRL), thyroxine (T 4), triiodothyronine (T 3) and corticosterone (B) were measured in the pigeon by RIA methods. 2. 2. Pigeons were maintained indoors under constant ambient temperature ( T a) and simulated natural daylight (LD), 12:12 L:D regimens or outdoors exposed to seasonal variations in temperature and photoperiod at Oulu, Finland. 3. 3. Only slight changes of gonadotropins (LH, FSH) were observed throughout the year, without any clear photosensitive or photorefractory period. In the indoor (natural LD) group, LH stayed elevated from May until October. 4. 4. Interdependence between the circannual hormonal fluctuation and photoperiod could not be shown, although the amplitude of FSH, T 4 and T 3 fluctuations of pigeons maintained in laboratory conditions were greater than that of natural LD and outdoor pigeons, whose circannual rhythms were similar. 5. 5. A high concentration of plasma PRL in autumn and the peak value of B in winter for all groups are thought to be correlated to lipid metabolism. 6. 6. Two peaks, the first in winter and the second in autumn, observed in T 4 and T 3 hormone profiles, may be due to molting of the pigeons.
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