Exposure of medicines to high temperatures in storage or in transit could reduce their efficacy, and most licences specify storage at 25 degrees C or less. To assess whether this criterion was being met, maximum temperatures in a general practice drug cupboard and in drug bags placed in car boots were recorded for two weeks during a British heatwave (average peak daily ambient temperature 26 degrees C). Also, ten neighbouring dispensing pharmacies were questioned about their temperature-control policies. On every day of the study, maximum temperatures in the drug cupboard and in the car boots exceeded 25 degrees C. Mean daily maxima (range) were: drug cupboard 30.7 (27.5-37.0); silver car 37.5 (32.0-43.5); dark blue car 41.8 (35.0-49.5). None of the local dispensaries had air conditioning or kept a temperature log. In the course of a British summer, medicines were exposed to temperatures that might in theory have reduced their efficacy. This aspect of quality control deserves more attention.