Bovine pineal gland extract was separated into fractions of different molecular weight via successive ultrafiltrations. The 1000–10 000 dalton fraction was purified by column chromatography yielding fractions A and B. The pineal fractions were tested for their ability to inhibit ovulation induced by PMSG and HCG in prepuberal mice. Administration of pineal fractions of molecular weights below 500 daltons, 500–1000 daltons, and fraction A (1000–10 000 daltons) suppressed the number of ovulations per mouse (P < 0.01). The 10 000–100 000 dalton fraction was the least potent (P < 0.10) of the inhibitory fractions and fraction B (1000–10 000 daltons) did not exhibit any inhibitory effect (P > 0.10). There was a positive linear relationship (P < 0.025) between the molecular weight of the inhibitory fractions administered and the proportion of mice that ovulated. All pineal extract fractions had a detrimental effect on body weight.