The purpose of this study was to determine the accuracy of transabdominal ultrasound in estimating the volume and weight of the canine prostate gland, to derive a simple formula from actual and ultrasonographic prostate measurements for estimating prostate volume and weight. and to evaluate the reproducibility of ultrasonographic measurements of linear prostate dimensions. The prostate volume and weight were measured and also calculated in 77 male canine cadavers (60 entire and 17 neutered) with the aid of statistical computer programmes using actual linear dimensions of the prostate ('actual') and those measured ultrasonographically before ('in vivo') and after ('in vitro') removal of the prostate gland. Therefore, 'in vivo', 'in vitro' and 'actual' estimations of prostate size were obtained and compared with prostatic volume and weight. The evaluation of the reproducibility of the ultrasonographic measurements made was performed in 59 of the dogs in which the prostate was imaged three times in longitudinal and three times in transverse sections. This study demonstrated that prostatic weight and volume are related to body weight and age in entire dogs but not in neutered dogs. Formulae were derived to estimate prostate weight and volume from physical and ultrasonographic measurements. There were good correlations between the ultrasonographic and physical measurements and prostate size derived from these measurements provided a good estimate of actual prostate size. 'Best subsets' analyses indicated that prostate width and length (measured physically or ultrasonographically) were the best predictors of prostatic volume and weight, both when all dogs were considered together and in entire dogs only. Statistical analyses indicated that there were no significant differences between the repeated measurements of length and depth (P>0.05). However, there was a significant difference between repeated measurements of width (P= 0.0.357).