To determine whether very short pulses of ultrasound (5 to 6 milliseconds) have less heat propagation in biological tissue (thermal inertia) than traditional pulses (50 milliseconds).Department of Ophthalmology and Visual Sciences, University of Utah Health Sciences Center, Salt Lake City, Utah.Thermal testing was done in balanced salt solution (BSS) and in eye-bank eyes. In the same fresh human eye-bank eye, net temperature increase after 20 seconds of ultrasound (50 milliseconds on and 50 milliseconds off) was compared with the increase after 6 milliseconds on and 12 milliseconds off with the same phacoemulsification unit. The same experiment and setting was run in BSS and the eye-bank ratios compared with the BSS ratios. Twenty runs were done at each power setting in BSS and 22 in the eye-bank eye.There was 10.9% less heat generated with 6-millisecond pulses of ultrasound in limbal tissue than in BSS compared with 50-millisecond pulses of ultrasound (P = .0002).Very short pulses of ultrasound (5 to 6 milliseconds) propagated less thermal energy in limbal tissue than in BSS compared with 50-millisecond ultrasound pulses.