SummaryTo contribute to the assessment of the degree of discomfort in rats after orbital puncture, we have examined the histological changes in the intraorbital tissues caused by this technique of blood sampling. Orbits were studied from rats euthanized either within 1 min, 4 days, 28 days or S6 days after puncture while under diethyl-ether anaesthesia. The techniques of 2 animal technicians were compared, one using a broken haematocrit capillary and the other using an intact Pasteur's pipette. Non-punctured orbits served as controls.Microscopic slides containing the eye in situ at 2 horizontal levels in the orbital region were examined for 37 parameters; the slides were scored blind and in random order. Orbital puncture caused haemorrhages in the puncture track and, depending on the technique used, also in the periosteum. Four days after puncture, inflammatory reactions were present in the puncture track. Depending on the technique of puncture, these reactions were also seen in the eye muscles and periosteum or in the Harderian gland. Within 4 weeks after puncture, the lesions had healed without detectable scars. The different histological effects of the·2 techniques of orbital puncture are discussed in the light of the characteristics of these techniques.