The effects of chemicals capable of antagonizing the functions of calmodulin, such as trifluoperazine, chlorpromazine, imipramine, alprenolol, W7, and W13, on the circadian conidiation rhythm of Neurospora crassa were examined. Trifluoperazine, at a 30-microM concentration, was most effective in shifting the phase of the conidiation rhythm and caused a maximum phase delay at circadian time (CT) 6 and maximum phase advance at CT 9. Chlorpromazine was less effective than trifluoperazine, and a 300-microM concentration of chlorpromazine was required for a similar phase shift. Imipramine, at a 1-mM concentration, caused only a small phase shift, while alprenolol had little effect on biological clock function. W7 and W13 caused phase delays longer than 10 hr at CT 6 and caused a phase advance of about 5 hr at CT 10 when present at a 200-microM concentration. However, W5 and W12, the dechlorinated homologues of W7 and W13, had no effects on clock function at the same concentration. Calmodulin was assayed by measurements of stimulation of cyclic nucleotide diphosphodiesterase activity. Calmodulin content remained constant in trifluoperazine-sensitive and trifluoperazine-insensitive phases for two cycles following the light-dark transition.