This study addresses a potentially general basis of measuring time in a biological timer. Here, we examined the effects of biogenic amines on the time-fixed post-copulatory sexually refractory stage (ca. 1 h) which is defined as the time interval between spermatophore protrusion and the onset of a calling or a mating response in the reproductive cycle of the male cricket. For subcuticular injection of amines (0.15 ml, 10(-2) mol l(-1)), the interval of the refractory stage was shortened by octopamine, serotonin, 5-hydoxytryptophan and N-acetyl-serotonin but was unchanged by tryptophan, melatonin or 5-hydroxyindol-3-acetic acid. The effect of 5-hydoxytryptophan was most potent (maximum shortening, 38%) and long lasting (ca. 4.5 h) while other amines effected only the injected cycle. Injection of 5-hydoxytryptophan (180 nl, 10(-2) mol l(-1)) into the terminal abdominal ganglion also decreased the interval to a similar extent. Simultaneous injection of 5-hydoxytryptophan with the inhibitor of the serotonin synthesis enzyme reduced the 5-hydoxytryptophan effect suggesting that this effect results from synthesis of serotonin from 5-hydoxytryptophan. The protein synthesis inhibitor cycloheximide had no effect on the interval. These results suggest that the reproductive timer is regulated by serotonergic neurons in the terminal abdominal ganglion without protein synthesis during the interval of the time-fixed sexually refractory stage.