Summary. The agronomic effectiveness of highly reactive North Carolina phosphate rock (NCPR), relative to triple superphosphate (TSP), was determined for individual harvests at 26 permanent pasture sites in the National Reactive Phosphate Rock Project from 1992 to 1995. The aim was to determine whether the performance of NCPR relative to TSP was consistent over the growing season. Poor performance of NCPR early in the growing season (early seasonal lags), and its subsequent improvement later in the season, was observed at 15 of the 26 sites in this project and for 20 of the 103 possible site x year combinations. At 3 additional sites the apparent early season lag in NCPR performance was followed by harvests where there was no phosphorus (P) response. At one P-leaching site with a long growing season the relative yields from NCPR plots increased as the year progressed and exceeded those from TSP late in th e season. The duration of the early season lag phase was associated with the rate of build-up in plant-available P over the 4-year duration of the trials. The lag disappeared after 1 or 2 years at sites with low P-sorbing, sandy soils where there was a significant build-up in Colwell P, but persisted for 4 years on high P-sorbing soils where limited increases in Colwell P occurred. The average early season yield reduction associated with use of NCPR, compared with using TSP, was 28%. Colwell soil test values alone did not provide a good indication of the likelihood of a seasonal effect occurring. The occurrence of an early season lag phase needs to be taken into account by farmers considering the use of RPR as reductions in feed supply at this time of the year have the potential to reduce stocking rates and farm profitability. The use of partially acidulated NCPR, or large applications of NCPR applied in the first year, were management strategies that minimised the occurrence of these seasonal effects. The suggestion that the early season lag effect is due to the inability of NCPR to meet P demands from newly establishing pasture legumes, and increased P requirements by pasture plants in cold conditions is also discussed.