There is much recent evidence that prolactin is an important influence on parental and incubatory behaviour in birds. In this study prolactin was measured at various stages of the breeding cycle in three closely related albatrosses (the wandering albatross Diomedea exulans, the grey-headed albatross D. chrysostoma, and the black-browed albatross D. melanophris). Each species is monogamous, laying one egg, with the sexes sharing parental duties, including lengthy incubation shifts. In experiments where blood samples were taken daily throughout single incubation shifts (of both sexes) and every 3 hr for 36 hr, high prolactin levels were observed, but there was no indication of any changes that might suggest direct relationships between the hormone concentrations and incubatory behaviour. However, high prolactin levels were characteristic of the whole incubation period with a significant decline in concentrations towards the end of the brood-guard period. The timing of the decline in prolactin levels remained constant, even when the incubation period was artificially lengthened or shortened, as did the overall duration of the incubation-brood-guard period. Further experiments eliminated the possibility that the secretion of prolactin was a response to tactile stimulation of the brood patch by the egg. These results suggest that the incubation period is not endogenously timed but that prolactin may still affect the overall duration of the incubation-brood-guard period, although having little or no effect on the number or duration of incubation shifts.