It is already known that the local treatment with Alphagan eye drops (a drug commonly used by glaucoma patients) reduces ocular blood flow. In-vitro studies suggest that the vasoconstrictive effect of Alphagan is more pronounced when this drug is combined with a prostaglandin analogue (eg. Xalatan eye drops). The investigators also question whether this effect is more pronounced in subjects suffering from a primary vascular dysregulation (PVD). To test for the effect of these drugs on PVD and non-PVD subjects the investigators will take measurements of corneal temperature as surrogate for ocular blood flow.

The investigators would like to investigate the effect of Alphagan eye drops, Xalatan eye drops and their combination (both Alphagan and Xalatan) on corneal temperature in healthy subjects with and without a primary vascular dysregulation (PVD).

The investigators would like to answer the following questions:

1. Do the individual drugs influence the circulation to the eye as quantified indirectly via corneal temperature?
2. Does the combination of Alphagan and Xalatan give an additive or even potentiated effect?
3. Is the sequence of application of drugs of relevance (Alphagan applied first, Xalatan second, or vice versa)?
4. Do PVD and non-PVD subjects respond differently to these drugs?

20 healthy non-PVD subjects and 20 healthy PVD subjects in the age range from 20-40 years will be recruited. Recruited subjects will have all measurements: corneal temperature, intraocular pressure (IOP), ear temperature, blood-pressure. Both prior to and after instillation of eye drops (Xalatan, Alphagan and placebo).

Inclusion Criteria:

• healthy
• age 18-48 years
• normal findings on ophthalmological examination

Exclusion Criteria:

• history of ocular or systemic disease
• chronic or current systemic or topical medication
• drug or alcohol abuse
• art. hypertension