BACKGROUND There is relatively little research on affective influences on physician behaviour, especially on prescribing and referrals. Affects include transitory moods and lasting emotions. OBJECTIVES We explored physician perceptions of the impact of four mood states on perceived rates of five behaviours: talking with patients, prescribing medications and referrals for laboratory tests, diagnostic tests and specialists. We also examined whether burnout modified the impact of moods on behaviour. METHODS A total of 188 family physicians responded anonymously to a self-reporting questionnaire that assessed the perceived rate of behaviours when in a positive, negative, tired and nervous mood and burnout level. RESULTS Five analyses of variance with repeated measures on mood states and contrast analyses computed the effects of mood and burnout on the behaviours. The mood factor was found significant for each of the behaviours, in all P < 0.001. The respondents reported that on good mood compared with negative mood days, they talked more, prescribed less and referred less. The burnout factor was also significant: high compared with low burnout physicians had higher perceived rates of all referral behaviours. Significant mood × burnout interactions indicated that the effects of mood were stronger among high compared with low burnout physicians. CONCLUSIONS The physicians perceived that their moods had different effects on different behaviours: the negative mood decreased talking and increased prescribing and referral behaviours and vice versa for the positive mood. Burnout intensified the effects of moods. The incremental effects of negative moods and burnout may impair quality of health care and may be costly to health services.