Zebrafish embryos were cultured in 50 μM compound in egg water or in egg water containing 1% DMSO from tailbud stage on. At the 48 hpf stage, embryos were individually and manually assessed for arrhythmia (A), formation of pericardiac or inflow tract edema (B), and atrial heart rates (C). (A) Eight out of the 90 drugs of the LOPAC Pfizer library produced arrhythmia in zebrafish embryos. DMSO-treated embryos never showed arrhythmia. n = 4–7 treatments with 31–56 embryos. (B) Seven drugs impaired overall heart function, resulting in edema formation. Dashed line indicates the percentage of embryos developing edema under DMSO treatment. n = 4–7 treatments with 25–83 embryos. (C) Tolterodine-treated embryos showed the highest variability in heart rate (shown as bpm) compared with DMSO-treated embryos (dashed line). Circles display heart rates of individual embryos. n = 10–158 embryos. Bar graphs display mean ± SEM, with circles indicating individual experiments. (A and B) Data analyzed by 1-way ANOVA, with *P < 0.05, **P < 0.01, and ****P < 0.0001.