Abstract: 9-(2-Hydroxyethoxy-methyl)-guanine (Acyclovir,ACV) was used as reagent in the UV-spectrophotometric determination of boron.In a buffer medium of NaOH-KCl solution at pH 12.0,a coordination complex was formed between borate and ACV,having its absorption maximum at the wavelength of 289 nm.Linearity range was kept from 0.31 to 31.2 mg·L-1 for boron.Detection limit (3s/k) found was 0.27 mg·L-1.The proposed method was used in the analysis of drug samples and urine samples for their boron contents.Tests for recovery were made by the standard addition method using these samples as matrixes,values of average recovery found were over 99%.