Ensuring the availability of essential drugs and using them appropriately are crucial if limited resources for health care are to be used optimally. While training of health workers throughout Zimbabwe in drug management (including stock management and rational drug use) resulted in significant improvements in a variety of drug use indicators, these achievements could not be sustained, and a new strategy was introduced based on the supervision of primary health care providers. This was launched in 1995 with a training course in supervisory skills for district pharmacy staff. In order to evaluate the impact of the supervision and the effectiveness of the training programme, adherence to standard treatment guidelines (STG) and stock management protocols was evaluated in a randomized controlled trial. The study compared three different groups of health facilities: those that received supervision for either use of STG (n = 23) or stock management (n = 21) - each facility acting as control for the other area of supervision - and a comparison group of facilities which received no supervision (n = 18). On-the-spot supervision by a specially trained pharmacy staff, based around identified deficiencies, took place at the start of the study and 3 months later. The evaluation compared performance on a variety of drug management indicators at baseline and 6-8 months after the second supervisory visit. The results of the study showed that, following supervision, overall stock management improved significantly when compared with the control and comparison groups. Similar improvements were demonstrated for adherence to STG, although the effect was confounded by other interventions. The study also showed that supervision has a positive effect on improving performance in areas other than those supervised, and demonstrated that pharmacy technicians with limited clinical skills can be trained to influence primary health care workers to positively improve prescribing practices. Allocating resources to supervision is likely to result in improved performance of health workers with regard to the rational use of essential drugs, resulting in improved efficiency and effectiveness.