The effects of intravenous injections of two different preparations of synthetic arginine vasotocin (AVT), deamino-dicarba-arginine vasotocin and arginine vasotocin diacetate tetrahydrate on serum levels of thyroxine (T4), triiodothyronine (T3) and melatonin were studied in the resting pigeon. Although variations depending on factors such as the type of AVT preparation and the dose used and also the duration of treatment were apparent, both preparations of AVT produced an increase in serum levels of T4 and a decrease in that of T3. The T3:T4 ratio was also significantly lower in response to AVT administration. However, no statistically significant changes in serum levels of melatonin were discernible with the administration of either of the two AVT preparations, even though melatonin levels in most AVT-treated groups showed a trend (P > 0.05) to be higher than that in the controls. The rise in T4 levels is indicative of increased release of T4 and/or the inhibition of its conversion to the active hormone, T3. The drop in T3 may account for the AVT-induced reduction in the metabolic rate of the pigeon observed in an earlier study. The absence of a significant increase in melatonin indicates that AVT could not produce such a change in the daytime in resting pigeons, in contrast to that in flown homing pigeons, suggesting that the modulating effect of AVT on melatonin levels is dependent on the physiological state of the animal.