Tumor-bearing pet dogs were treated with three weekly doses of intravesical PNP (1 mg/20 ml paclitaxel-equivalent). Tumor samples were obtained immediately after the first treatment, and again on day 7 and day 14. The day 7 and 14 samples were taken before animals received their second and third weekly treatments. Hence, the day 7 samples were obtained 1 week after the first treatment, and the day 14 samples were obtained 2 weeks after the first treatment and 1 week after the second treatment. Drug concentrations, measured by ELISA, were 36.3±29.4 μg/g at 2 hr after initiation of treatment (mean±SD; median was 28.8 μg/g; range was 6.4-76.6 μg/g; n=7), 43.9±35.9 μg/g at 1 week after initiation of the first treatment (mean±SD; median was 30.3 μg/g; range was 16.8-117.6 μg/g; n=6), and 35.5±37.8 μg/g at 2 weeks after initiation of the first treatment and 1 week after the second treatment (mean±SD; median was 19.7 μg/g; range was 5.5-98.8 μg/g; n=6). Solid lines: mean values. Dotted lines: median values.