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Lead II electrocardiograms (ECGs) were recorded in 88 dogs ranging in weight from 3 to 50 kg. A commercial direct-writing electrocardiograph was used to record the ECGs, first with manual HUM filter (notch filter at 50/60 Hz) and EMG filter (cut-off frequency of -3 dB at 35 Hz) on, immediately followed by a recording with both filters off. The mean (sd) reduction in R-amplitude with the filters on was 53(18) per cent with a range from 22 to 100 per cent and a median value of 51 per cent. The R-amplitude with the filters off was related to the R-amplitude with the filters on, making it possible to correct for the effect of the filters. The reduction in R-amplitude was inversely correlated with the weight of the dog and with the duration of the QRS-complex. Other known changes induced by filters, such as the elimination of notches and the slurring of the junction between the QRS-complex and the ST-interval were also observed. The inverse relationship between the weight of the dog and the magnitude of the reduction in R-amplitude by the filters indicates that ECG machines with different characteristics should ideally be used to record ECGs in dogs of different sizes.