First-pass nuclear angiocardiography (FPNA) was performed in 5 normal dogs and in 19 dogs with left-to-right shunting patent ductus arteriosus (PDA). Affected dogs were evaluated before and after an occlusion procedure (surgical ligation or transarterial coil embolization). Two methods of analysis were used to evaluate the pulmonary time-activity curves and assess the magnitude of the shunt. One method involved the calculation of a simple count ratio (C2: C1). The other method used gamma variate modeling to estimate the ratio of pulmonary to systemic blood flow (QP:QS). Values for normal dogs (mean ± SD) were C2:C1 = 0.39 ± 0.11 and QP:QS = 1.10 ± 0.03. Values for affected dogs were C2:C1 = 0.59 ± 0.17, QP:QS = 2.01 ± 0.46 before intervention and QP:QS = 1.21 ± 0.17, C2:C1 = 0.42 ± 0.12 after intervention. Both methods detected significant differences between normal and affected dogs (QP:QS, t-test, P < .001; C2:C1; t-test; P= .018) and between affected dogs pre- and postocclusion (QP: QS, paired t-test, P < .001; C2 : C1, paired t-test, P= .002). However, QP : QS was a better separator of normal from affected animals. First-pass nuclear angiocardiography, by the gamma variate method, may be a useful, adjunct diagnostic test in evaluation of animals with left-to-right shunting PDA, particularly in animals with residual shunting after an occlusion procedure.