Diode laser stimulation provides uniform heating of tissue and cells, allowing a margin of laser power and duration between non-invasive activation of nociceptors and damage*. Method: Laser pulses: 20-50ms, 0.1-10W were used to stimulate in vitro: excised patches from HEK293 cells (TRPV1 expressing, n=9; control, n=1) and rat DRG cells expressing either TRPV1 (n=8) or TRPV2 (n=3); and in vivo: C-fibers of rat saphenous nerve(SN) trunk. Diode laser pulses were applied by optical fiber. Cell activation was recorded and analyzed using standard patch clamp methods. SN was also stimulated electrically with bipolar electrodes. TRPV1 and TRPV2 were identified in DRG neurons by immunocytochemistry. Laser induced temperature was assessed by comparison of open pipette laser current with current evoked by heated solutions (TCbip HPRE2). Results: Laser pulses allowed reproducible brief stimulation of heat-gated ion channels. The lowest STD of laser-induced current in HEK293 cells was 0.12 (ten stimuli; 50ms, 4W, 10s interval). In TRPV1 positive DRG neurons, currents were rapid, highly uniform and saturation was reproducible. Activation and peak currents were achieved at estimated peak temperatures of 55°C and 70°C. Threshold activation was also observed in DRG neurons expressing TRPV2. Conduction velocity for laser-activated saphenous afferents was in the C-fiber range 0.5-1m/s (20ms/15W; 300ms/3 W); conduction time of electrical stimulation 0.3-30m/s. The measured absorption coefficients of both bath solution and mineral oil were very similar (0.04 ±0.002cm-1). Conclusions: These results provide evidence that diode laser pulses are reproducible, non-invasive stimuli that may allow brief, uniform activation of TRPV1 in vivo and in vitro. Numerous drug discovery and evaluation applications are possible. It is likely that laser stimuli may be useful for activation of other TRP proteins as well. *Nemenov MI, Mikkelson J. Limitations of laser application in pain research. 9thWorld Congress on Pain Veinna, 1999.