Objective: To investigate the effectiveness of Mitchell's technique including diaphragmatic breathing, compared with diaphragmatic breathing alone and supine lying. Method: Forty-five normal male subjects were randomly assigned to three groups: (1) Mitchell with diaphragmatic breathing; (2) diaphragmatic breathing alone; (3) the control condition of supine lying. Each subject underwent one 18-minute treatment session. Heart rate was measured using a pulse oximeter, at baseline, and before and after intervention. Results: Results were analysed using a two-way mixed factorial ANOVA with Sheffé's post hoc test. Before intervention there was no interaction effect and no significant difference between the groups at baseline and pre-intervention measurements (p > 0.05). However, the supine position was shown to reduce heart rate significantly in all groups. After intervention a significant reduction in heart rate was shown in groups 1 (p < 0.05) and 2 (p < 0.01), although no differences were found between the two groups (NS). Both treatment groups reduced heart rate to a significantly greater extent than supine lying. Conclusion: Diaphragmatic breathing with and without Mitchell's relaxation technique significantly reduced heart rate. The lack of significant difference between the two treatment groups suggests that Mitchell's relaxation technique may be only as effective as diaphragmatic breathing alone. It may be more physiologically justifiable to use diaphragmatic breathing rather than Mitchell's complete technique. This is one of the first studies to suggest that diaphragmatic breathing can be used alone, and need not be incorporated into generalised relaxation procedures. New recommendations are made for future research.