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Hamner (1963, 1966) has demonstrated that the photoperiodic response of birds depends on a circadian cycle within which occur limited phases of sensitivity to photic stimulation. Lofts & Murton (1968) speculated that different gonadotrophins might each be secreted according to different circadian phases. To test this possibility male greenfinches Chloris c. chloris, which were caught wild between late December and early January when their testes were regressed, were held on natural photoperiods until 24 January. They were then divided into six groups and artificial lighting was provided between 07.00–14.00 hr. for 14 days, when seven birds were killed as controls (group A). Following this entrainment, the remaining groups were given 6 hr. of light between 07.00–13.00 hr. and in addition a further 1 hr. as shown in Fig. 1. After a further 19 days all birds were killed between 10.00–11.00 hr., an arbitrary time which does not necessarily reflect the