1. Spiral strips of human digital arteries have been studied in vitro to investigate whether dl-propranolol, d-propranolol, oxprenolol and labetalol have peripheral vascular effects in man.
2. Labetalol was a potent inhibitor of contractile responses to noradrenaline, but had less effect on responses to 5-hydroxytryptamine and barium chloride.
3. dl-and d-propranolol were equally effective inhibitors of responses to barium chloride. They were only weak antagonists of noradrenaline responses, but stronger, non-competitive antagonists of 5-hydroxytryptamine responses.
4. Oxprenolol was only a weak inhibitor of the responses to both noradrenaline and 5-hydroxytryptamine and had little effect on responses to barium chloride.
5. It is concluded that labetalol has specific α-adrenoreceptor-blocking properties, which are probably relevant to its therapeutic action in man. Propranolol has non-specific inhibitory effects on vascular smooth muscle, which might contribute to its hypotensive activity at high concentrations, but oxprenolol has only slight peripheral effects that are probably therapeutically insignificant.