The photoplethysmographic signal (PPG) from forearm skin and the ECG were recorded simultaneously from healthy subjects. The optical signal was derived with a fiber-optic probe which consisted of 61 fiber pairs. The peak-to-peak averaged AC-signal was calculated. The dependency of this signal on light wavelength and on sample volume was studied. The light intensities used at 560 nm were 0.015 and 0.029 mWmm-2 and at 940 nm we used 0.029, 1.37 and 2.77 mWmm-2. A theoretical model for calculation of the sample volumes was also developed. The amplitude of the AC-signal at 560 nm is larger than at 940 nm at the same intensity (0.029 mWmm-2). At 560 nm and small sample volumes (7 fiber pairs) the amplitude did not increase with increased intensity. At 940 nm using 1.37 and 2.77 mWmm-2 the AC-signal increased with the number of fiber pairs (volume).© (1995) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.