Isolates of the variant avian paramyxovirus type 1 virus causing disease in racing and show pigeons showed a gradual but statistically significant diminution of virulence for chickens, as measured by intracerebral pathogenicity index (ICPI) tests in one-day-old chicks, over the period July 1983-June 1985. Six isolates were passaged through two-week-old chickens by intramuscular inoculation. Three of these isolates showed an increase in virulence for six-week-old chickens, as measured by intravenous pathogenicity index (IVPI) tests. Indices of 0.00 for two of these isolates and 0.34 for the other rose to over 2.00 after three passages. The two viruses with initially low ICPI values of 0.85 and 1.08 failed to show an increase in IVPI even after six passages through two-week-old chickens. However the two viruses both showed increases in ICPI values after six passages to 1.47 and 1.37 respectively. The other virus, with an initial ICPI of 1.51, showed no increase in ICPI value after six passages in chickens, but an increase in IVPI value from 0.00 to 0.53 was recorded.