During transport, broiler chickens experience motion and vibration at frequencies from 0β’5 to 25 Hz, some of which they are likely to find aversive. The aversion of chickens to different frequencies (0β’5-10 Hz) and rootmean-square accelerations (0-5 m/s) of vertical and horizontal vibration was tested using a passive-avoidance, operant-conditioning procedure. Chickens were trained to receive their daily food ration by making a pre-selected, random number of pecks (between 1 and 39) at a computer-controlled, operant panel for repetitive small food rewards during a fixed 2 h period each day. Additionally, every 20th peck initiated 30 s of motion with pre-determined characteristics. Thus, they were in conflict in their motivations to acquire their daily food ration and to avoid initiating the motion. The pecking rate in the experimental period measured the degree of aversion of the chickens to the imposed vibration. Each chicken was studied for zero acceleration and one other value. Four chickens were used in a completely randomized design for each of the 15 horizontal and 16 vertical motions. Models to predict pecking rate from vibration frequency and root-mean-square acceleration were obtained using non-linear regression analysis for vertical and horizontal motions independently with more than 98β’5% of the variance accounted for. These models were used to determine families of contours of equal pecking rates. Each contour joins co-ordinates of frequency and acceleration which the chickens find equally aversive. The higher root-mean-square accelerations and the lower frequencies were most aversive. The chosen models give a linear response with the effect of frequency independent of acceleration magnitude. Thus, the effects of different vibration frequencies on the chickens are accounted for by frequency weightings derived from the inverse of any one of the contours. The frequency weighting gives the relative value at each frequency by which the acceleration magnitude must be multiplied to weight it according to the level of chicken aversion at that frequency. At frequencies of f Hz, the weightings are 1β’66/ f for vertical motion and 1β’303/ f for horizontal motion. Root-mean-square weighted accelerations derived from these frequency weightings may be used to predict the likely level of broiler chicken aversion within the scale of 0β’16-0β’315 m/s indicating ''not aversive'', 0β’5-1β’00 m/s ''aversive'' and 2β’0-4β’0 m/s ''extremely aversive''.