Herbicides tested included simazine, atrazine, propazine, prometryne and ipazine. Plainfield sand was placed in greenhouse flats to a 2-in. depth and the appropriate herbicide sprayed on the soil surface at 2, 4, 8 or 16 1b./acre. The soil and herbicide were then mixed in a Patterson-Kelly twin shell blender and distributed into cylindrical, one-pint paper cartons with holes in the bottom for drainage. One hundred Pinus resinosa seeds were planted in each of 10 cartons for each dosage of each herbicide. Ten cartons of mixed soil planted with seed but without added herbicide served as controls. The cartons were placed on a greenhouse bench and watered daily. Toxicity symptoms and survival counts were noted periodically. 110 days after planting the living seedlings were counted and their tops clipped off at the ground line for dry weight determinations Table 1. MEAN DRY WEIGHTS OF SHOOTS OF LIVE SEEDLINGS 110 DAYS AFTER HERRICIDE TRAEATMENT(PERCENT OF CONTROL);Dosage t (Ib./acre) Atrazine Simazine Herbicide Prometryne Propazine Ipazine 2 0* 12-2* 19-8* 81-0 100'Q4 7-9* 8-5* 17-9* 31-7* 138-3 8 0-2* 5-3* 0* 12-8* 75-616 0-2* 0* 0* 0-2* 21-6* * Sig. diff, from control at 1 per cent level.? Herbicide applied to soil surface at indicated rate and then incorporated into soil. In general, toxicity to pine seedlings varied greatly among herbicides with atrazine the most toxic and ipazine the least. Within 10 days after germination variable herbicide toxieity was evident, with atrazine, simazine, and prometryne injury already apparent at the higher dosages. Neither ipazine nor propazine injury was observed at this early stage. Early toxieity symptoms included slight needle curling, chlorosis, and growth inhibition. The symptoms became more severe with time and later were apparent at low dosages of atrazine, prometryne and simazine. All, or virtually all, seedlings were killed within 110 days after planting by the two highest dosages of atrazine and prometryne and the highest dosages of simazine and propazine (Table 1). Atrazine was highly toxic at all dosages, while prometryne was far more toxie at the two highest dosages than at the two lowest ones. Propazine was only mildly toxic at low dosages but very toxic at the highest dosage. Ipazine was very toxic only at the highest dosage. Especially interesting was the accelerated late mortality under Certain treatments. Ipazine, for example, caused no readily apparent damage at any dosage 80 days after planting. In the next 20 days, however, toxicity symptoms developed very rapidly in plants treated with ipazine at the highest dosage. This emphasizes the importance of investigating herbicide toxicity over relatively long periods of time. This would seem to be especially important in the case of triazine compounds since they do not leach or degrade rapidly3'4.