Attempts to induce carrying of nest material in isolated male Bengalese finches by injecting testosterone, oestrogen, progesterone or prolactin were ineffective. Neither prolactin nor progesterone in the doses used appeared to affect incubation behaviour. There was a suggestion that prolactin depressed song numbers. Inexperienced birds are in general slower than experienced ones in settling down and starting to build. Monosexual male pairs start to carry more quickly when both partners have been kept in the same small group before pairing, than when they are from different groups. Most birds with previous breeding experience and which had been in the same group as their partners before pairing began to carry at a high rate immediately after pairing. The slight increase in carrying in the few days thereafter is probably attributable to the need to settle down in the new situation and to practice making a higher rate of carrying possible. It is probable that the hormonal state is appropriate for carrying at the time of pairing but that the 75% of males which do not carry in isolation are only stimulated to do so by the presence of a known mate, a nest site and an uncompleted nest. The main control of carrying thus appears to lie with external stimuli acting directly on the nervous system rather than through the meditation of endocrine changes. In the discussion it is suggested that direct effects of external stimuli may be more important in inducing long term behaviour changes during reproduction than has previously been supposed.