Temporal changes of circulating serum hormones were measured to compare the reproductive endocrinology of laying and nonlaying mallards. In this study all sixteen control mallards left with their mates laid eggs, while only one of sixteen mallards stressed by daily movement into new pens, laid eggs. Serum levels of luteinizing hormone (LH), prolactin, estradiol, and progesterone were significantly lower (P less than 0.05) in stressed nonlaying mallards than in laying mallards over the 7-week period. Within 1 week of the rotation treatment, LH concentrations in stressed mallards averaged (means +/- SEM) 2.72 +/- 0.19 ng/ml and were significantly lower (P less than 0.05) than LH levels in the controls (3.62 +/- 0.18 ng/ml). After 7 weeks, injections of luteinizing hormone releasing hormone (LHRH) induced a greater change in circulating LH levels in stressed mallards (2.1 +/- 0.3 ng/ml) than in breeding control mallards (0.9 +/- 0.2 ng/ml). These data demonstrate that the lack of reproduction in stressed mallards was associated with LHRH-sensitive pituitary pools of LH, despite their low concentrations of serum LH. These data suggest that the block in reproduction is a failure of the hypothalamus to produce or release releasing hormones. The serum hormone levels of the control mallards varied temporally with stages in the nesting cycle. LH levels increased with the onset of nesting activity, and showed marked fluctuations during the laying period. LH levels fell at the onset of incubation but increased after loss of clutch. Estradiol levels were highest prior to the laying of the first egg and their peak coincided with the initial nest building behavior of the females. Progesterone levels increased sharply with the laying of the 2nd-4th eggs, decreased sharply with the laying of the 6th egg, and then increased slightly at the end of the nesting cycle. Prolactin levels were initially low but gradually increased with laying and incubation activity, declined with loss of clutch, and increased again with renesting activity. Prolactin levels in the stressed mallards also increased (P less than 0.01) over the 7-week period, but significantly less (P less than 0.05) than in layers.