Variability in 408-MHz flux density between 1992 March and 1993 February/March has been investigated using the DRAO Synthesis Telescope for a complete sample of 248 radio sources with S_408MHz>100 mJy; the variability properties of the sample have been monitored at 151 MHz over the past 10 yr and simultaneous observations were carried out at 151 MHz. 408-MHz variations at a level >=10 per cent could have been detected in ~=50 of these sources, but none was found. Only one source in the entire sample, B1044+4524, shows significant variability; its flux density decreased by 6 per cent over the 1-yr period. Although it had previously been found to be a 151-MHz variable, it varied by less than 2 per cent at 151 MHz over the same period. Variability over a period of 15 yr was also investigated by comparing the DRAO data with those from the B3 catalogue for 134 sources with S_408MHz>200 mJy. Only two sources exhibit significant variations, one of them again being B1044+4524 the flux density of which decreased by 13 per cent; its 151-MHz flux density decreased by 19 per cent over the same period.