In clinical practice the authors have found the 2-mm wide Periopaper slightly too large for insertion into the gingival crevice. This study investigated the reliability of Periotron readings for quantitation of gingival fluid when using filter paper strips which are narrower than the Periopaper. The round-ended strips were cut to a standard size 1 mm wide and 15 mm long using a conchotome. A microsyringe was used to deposit 0.1 microliter of four different fluids (distilled water, undiluted horse serum, diluted horse (1:2) and human (1:2) sera) onto Periopaper and the narrow S-690 strips. The moistened end of each strip was placed between the jaws of both the Periotron 600 and the Periotron 6000, and the maximum digital reading was recorded in two series of 20 measurements for each of the four fluids and two strips, giving a total of 640 assessments. A comparison between the two series confirmed the excellent reproducibility of Periotron readings. The comparison between strips revealed from 10% (Periotron 600) to 20% (Periotron 6000) lower readings for the S-690 than for the Periopaper strips for all fluids. The SD of the readings for both the S-690 and the Periopaper strip was low enough to make the difference between strips statistically significant. The slightly lower readings obtained with the narrow S-690 strip were found as reliable as those recorded when using Periopaper.