Ninety-six hospitalized insomniac patients were asked to compare by a daily questionnaire the effect on sleep of the colour of the capsules and the order of administration of two treatments--heptabarbital and placebo. A balanced incomplete-block design was used; sleep onset time and sleep duration time were the main criteria of assessment. There was a significant interaction of colour and order of administration with sex. The placebo response was positive in 36% of instances.