The ultrasonic pachometer has been introduced recently into our laboratory for use in the eye-irritation test using enucleated rabbit eyes. We have compared the performance of this kind of pachometer with that of the optical pachometer. The results for corneal swelling, reported by Burton et al. (1981) and obtained using an optic pachometer, were compared with those obtained using an ultrasonic pachometer to test the same 11 substances. The mean corneal swelling 240 min after application of the test substances to four eyes resulted in a very good correlation (r = 0.984) between the optic and ultrasonic pachometer data. A good Spearman rank correlation (r = 0.906; P < 0.0005) was also found between corneal swelling and opacity. The use of the ultrasonic pachometer requires the contact of the ultrasonic probe tip with the corneal epithelium. Although epithelial damage was observed due to probe contact, we were not able to demonstrate that this would influence corneal swelling. The ultrasonic technique is a considerable improvement over the optic technique because: